using AutoMapper;
using Marco.Pms.DataAccess.Data;
using Marco.Pms.Helpers.Utility;
using Marco.Pms.Model.Dtos.Collection;
using Marco.Pms.Model.Dtos.PurchaseInvoice;
using Marco.Pms.Model.Employees;
using Marco.Pms.Model.Entitlements;
using Marco.Pms.Model.Filters;
using Marco.Pms.Model.MongoDBModels.Utility;
using Marco.Pms.Model.OrganizationModel;
using Marco.Pms.Model.Projects;
using Marco.Pms.Model.PurchaseInvoice;
using Marco.Pms.Model.ServiceProject;
using Marco.Pms.Model.Utilities;
using Marco.Pms.Model.ViewModels.Activities;
using Marco.Pms.Model.ViewModels.Collection;
using Marco.Pms.Model.ViewModels.Organization;
using Marco.Pms.Model.ViewModels.Projects;
using Marco.Pms.Model.ViewModels.PurchaseInvoice;
using Marco.Pms.Services.Extensions;
using Marco.Pms.Services.Service.ServiceInterfaces;
using MarcoBMS.Services.Service;
using Microsoft.EntityFrameworkCore;
using System.Text.Json;
using Document = Marco.Pms.Model.DocumentManager.Document;

namespace Marco.Pms.Services.Service
{
    public class PurchaseInvoiceService : IPurchaseInvoiceService
    {
        private readonly IDbContextFactory<ApplicationDbContext> _dbContextFactory;
        private readonly IServiceScopeFactory _serviceScopeFactory;
        private readonly ILoggingService _logger;
        private readonly S3UploadService _s3Service;
        private readonly IMapper _mapper;

        private readonly Guid DraftInvoiceStatusId = Guid.Parse("8a5ef25e-3c9e-45de-add9-6b1c1df54381");
        private readonly Guid DeliveryChallanTypeId = Guid.Parse("ca294108-a586-4207-88c8-163b24305ddc");
        public PurchaseInvoiceService(IDbContextFactory<ApplicationDbContext> dbContextFactory,
            IServiceScopeFactory serviceScopeFactory,
            ILoggingService logger,
            S3UploadService s3Service,
            IMapper mapper)
        {
            _dbContextFactory = dbContextFactory;
            _serviceScopeFactory = serviceScopeFactory;
            _logger = logger;
            _s3Service = s3Service;
            _mapper = mapper;
        }

        #region =================================================================== Purchase Invoice Functions ===================================================================
        /// <summary>
        /// Retrieves a paged list of purchase invoices for a given tenant with support for
        /// advanced filters, project-based search across Infra and Service projects, and
        /// consistent structured logging and error handling.
        /// </summary>
        /// <param name="searchString">Optional basic search string (currently not used, kept for backward compatibility).</param>
        /// <param name="filter">JSON string containing advanced filter configuration (list/date/search/group filters).</param>
        /// <param name="isActive">Flag to filter active/inactive invoices.</param>
        /// <param name="pageSize">Number of records per page.</param>
        /// <param name="pageNumber">Current page number (1-based).</param>
        /// <param name="loggedInEmployee">Currently logged-in employee context.</param>
        /// <param name="tenantId">Tenant identifier for multi-tenant isolation.</param>
        /// <param name="ct">Cancellation token for cooperative cancellation.</param>
        /// <returns>Standard ApiResponse containing a paged invoice list payload.</returns>
        public async Task<ApiResponse<object>> GetPurchaseInvoiceListAsync(string? searchString, string? filter, bool isActive, int pageSize, int pageNumber, Employee loggedInEmployee, Guid tenantId,
            CancellationToken ct)
        {
            // Check if the employee has the necessary permissions
            var viewAllPermissionTask = HasPermissionAsync(PermissionsMaster.ViewAllPurchaseInvoice, loggedInEmployee.Id);
            var viewSelfPermissionTask = HasPermissionAsync(PermissionsMaster.ViewSelfPurchaseInvoice, loggedInEmployee.Id);

            var viewAllPermission = await viewAllPermissionTask;
            var viewSelfPermission = await viewSelfPermissionTask;

            if (!viewAllPermission && !viewSelfPermission)
            {
                _logger.LogWarning("Access Denied: {EmployeeId} do not have permission to view purchase invoice list", loggedInEmployee.Id);
                return ApiResponse<object>.ErrorResponse("You do not have permission to view purchase invoice list.", "You do not have permission to view purchase invoice list.", 403);
            }

            // Basic argument validation and guard clauses
            if (tenantId == Guid.Empty)
            {
                _logger.LogWarning("GetPurchaseInvoiceListAsync called with empty TenantId. EmployeeId: {EmployeeId}", loggedInEmployee.Id);

                return ApiResponse<object>.ErrorResponse("Tenant information is missing. Please retry with a valid tenant.", 400);
            }

            if (pageSize <= 0 || pageNumber <= 0)
            {
                _logger.LogWarning(
                    "GetPurchaseInvoiceListAsync called with invalid paging parameters. TenantId: {TenantId}, EmployeeId: {EmployeeId}, PageSize: {PageSize}, PageNumber: {PageNumber}",
                    tenantId, loggedInEmployee.Id, pageSize, pageNumber);

                return ApiResponse<object>.ErrorResponse("Invalid paging parameters. Page size and page number must be greater than zero.", 400);
            }

            // A correlationId can be pushed earlier in middleware and enriched into all logs.
            // Here it is assumed to be available through some context (e.g. _requestContext).

            _logger.LogInfo(
                "GetPurchaseInvoiceListAsync started. TenantId: {TenantId}, EmployeeId: {EmployeeId}, IsActive: {IsActive}, PageSize: {PageSize}, PageNumber: {PageNumber}",
                tenantId, loggedInEmployee.Id, isActive, pageSize, pageNumber);

            try
            {
                await using var context = await _dbContextFactory.CreateDbContextAsync(ct);

                // Base query for purchase invoices scoped to tenant and active flag
                IQueryable<PurchaseInvoiceDetails> query = context.PurchaseInvoiceDetails
                    .Include(pid => pid.Organization)
                    .Include(pid => pid.Supplier)
                    .Include(pid => pid.Status)
                    .Where(pid => pid.IsActive == isActive && pid.TenantId == tenantId);

                var advanceFilter = TryDeserializeFilter(filter);

                // Apply filters based on employee permissions
                if (!viewAllPermission && viewSelfPermission)
                {
                    query = query.Where(pid => pid.CreatedById == loggedInEmployee.Id);
                }

                // Apply ordering, default sort, etc. through your custom extension
                query = query.ApplyCustomFilters(advanceFilter, "CreatedAt");

                if (advanceFilter != null)
                {
                    // Apply list / dropdown / enum filters
                    if (advanceFilter.Filters != null)
                    {
                        query = query.ApplyListFilters(advanceFilter.Filters);
                    }

                    // Apply created/modified date range filters
                    if (advanceFilter.DateFilter != null)
                    {
                        query = query.ApplyDateFilter(advanceFilter.DateFilter);
                    }

                    // Apply non-project search filters on invoice fields
                    if (advanceFilter.SearchFilters != null)
                    {
                        // NOTE: fixed logic - use && so that Project/ProjectName are excluded
                        var invoiceSearchFilter = advanceFilter.SearchFilters
                            .Where(f => f.Column != "ProjectName" && f.Column != "Project")
                            .ToList();

                        if (invoiceSearchFilter.Any())
                        {
                            query = query.ApplySearchFilters(invoiceSearchFilter);
                        }
                    }

                    // Apply grouping if configured
                    if (!string.IsNullOrWhiteSpace(advanceFilter.GroupByColumn))
                    {
                        query = query.ApplyGroupByFilters(advanceFilter.GroupByColumn);
                    }
                }

                bool isProjectFilter = false;

                // Run project lookups in parallel to reduce latency for project search scenarios.
                // Each task gets its own DbContext instance from the factory.
                var infraProjectTask = Task.Run(async () =>
                {
                    await using var projContext = await _dbContextFactory.CreateDbContextAsync(ct);

                    IQueryable<Project> infraProjectsQuery = projContext.Projects
                        .Where(p => p.TenantId == tenantId);

                    if (advanceFilter?.SearchFilters != null && advanceFilter.SearchFilters.Any())
                    {
                        var projectSearchFilter = advanceFilter.SearchFilters
                            .Where(f => f.Column == "ProjectName" || f.Column == "Project")
                            .Select(f => new SearchItem { Column = "Name", Value = f.Value })
                            .ToList();

                        if (projectSearchFilter.Any())
                        {
                            infraProjectsQuery = infraProjectsQuery.ApplySearchFilters(projectSearchFilter);
                            isProjectFilter = true; // NOTE: shared flag, see comment below.
                        }
                    }

                    return await infraProjectsQuery
                        .Select(p => _mapper.Map<BasicProjectVM>(p))
                        .ToListAsync(ct);
                }, ct);

                var serviceProjectTask = Task.Run(async () =>
                {
                    await using var projContext = await _dbContextFactory.CreateDbContextAsync(ct);

                    IQueryable<ServiceProject> serviceProjectsQuery = projContext.ServiceProjects
                        .Where(sp => sp.TenantId == tenantId);

                    if (advanceFilter?.SearchFilters != null && advanceFilter.SearchFilters.Any())
                    {
                        var projectSearchFilter = advanceFilter.SearchFilters
                            .Where(f => f.Column == "ProjectName" || f.Column == "Project")
                            .Select(f => new SearchItem { Column = "Name", Value = f.Value })
                            .ToList();

                        if (projectSearchFilter.Any())
                        {
                            serviceProjectsQuery = serviceProjectsQuery.ApplySearchFilters(projectSearchFilter);
                            isProjectFilter = true; // This is safe for bool but can be refactored for purity.
                        }
                    }

                    return await serviceProjectsQuery
                        .Select(sp => _mapper.Map<BasicProjectVM>(sp))
                        .ToListAsync(ct);
                }, ct);

                await Task.WhenAll(infraProjectTask, serviceProjectTask);

                var projects = infraProjectTask.Result ?? new List<BasicProjectVM>();
                if (serviceProjectTask.Result != null && serviceProjectTask.Result.Any())
                {
                    projects.AddRange(serviceProjectTask.Result);
                }

                // If project filters were involved, constrain invoices to those projects
                if (isProjectFilter && projects.Any())
                {
                    var projectIds = projects.Select(p => p.Id).ToList();
                    query = query.Where(pid => projectIds.Contains(pid.ProjectId));
                }

                if (!string.IsNullOrWhiteSpace(searchString))
                {
                    query = query.Where(pid => pid.Title.Contains(searchString) ||
                    (pid.UIDPrefix + "/" + pid.UIDPostfix.ToString().PadLeft(5, '0')).Contains(searchString));
                }
                // Compute total count before paging
                var totalCount = await query.CountAsync(ct);

                // Apply paging
                var purchaseInvoices = await query
                    .Skip((pageNumber - 1) * pageSize)
                    .Take(pageSize)
                    .ToListAsync(ct);

                var totalPages = totalCount == 0
                    ? 0
                    : (int)Math.Ceiling((double)totalCount / pageSize);

                // Map invoice entities to view model and attach project details
                var response = purchaseInvoices.Select(pi =>
                {
                    var vm = _mapper.Map<PurchaseInvoiceListVM>(pi);
                    vm.Project = projects.FirstOrDefault(p => p.Id == pi.ProjectId);
                    return vm;
                }).ToList();

                var pagedResult = new
                {
                    CurrentPage = pageNumber,
                    PageSize = pageSize,
                    TotalPages = totalPages,
                    TotalCount = totalCount,
                    HasPrevious = pageNumber > 1,
                    HasNext = pageNumber < totalPages,
                    Data = response
                };

                _logger.LogInfo(
                    "GetPurchaseInvoiceListAsync completed successfully. TenantId: {TenantId}, EmployeeId: {EmployeeId}, TotalCount: {TotalCount}, ReturnedCount: {ReturnedCount}, PageNumber: {PageNumber}, PageSize: {PageSize}",
                    tenantId, loggedInEmployee.Id, totalCount, response.Count, pageNumber, pageSize);

                var successMessage = totalCount == 0
                    ? "No purchase invoices found for the specified criteria."
                    : "Purchase invoice list fetched successfully.";

                return ApiResponse<object>.SuccessResponse(pagedResult, successMessage, 200);
            }
            catch (OperationCanceledException ocex)
            {
                // Respect cancellation and return a 499-style semantic code if your ApiResponse supports it
                _logger.LogError(ocex, "GetPurchaseInvoiceListAsync request was canceled. TenantId: {TenantId}, EmployeeId: {EmployeeId}", tenantId, loggedInEmployee.Id);

                return ApiResponse<object>.ErrorResponse("The request was canceled by the client or the server. Please retry if this was unintentional.", 499);
            }
            catch (DbUpdateException dbEx)
            {
                _logger.LogError(dbEx,
                    "Database error while fetching purchase invoices. TenantId: {TenantId}, EmployeeId: {EmployeeId}", tenantId, loggedInEmployee.Id);

                return ApiResponse<object>.ErrorResponse("A database error occurred while fetching purchase invoices. Please try again later or contact support.", 500);
            }
            catch (Exception ex)
            {
                _logger.LogError(ex, "Unhandled exception in GetPurchaseInvoiceListAsync. CorrelationId: {CorrelationId}, TenantId: {TenantId}, EmployeeId: {EmployeeId}", tenantId, loggedInEmployee.Id);

                return ApiResponse<object>.ErrorResponse("An unexpected error occurred while fetching purchase invoices. Please try again later or contact support.", 500);
            }
        }

        /// <summary>
        /// Retrieves the details of a specific purchase invoice, including project details and S3 attachment links.
        /// </summary>
        /// <param name="id">The unique identifier of the Purchase Invoice.</param>
        /// <param name="loggedInEmployee">The employee requesting the data.</param>
        /// <param name="tenantId">The tenant identifier for data isolation.</param>
        /// <param name="ct">Cancellation token for async operations.</param>
        /// <returns>A wrapped response containing the Purchase Invoice View Model.</returns>
        public async Task<ApiResponse<PurchaseInvoiceDetailsVM>> GetPurchaseInvoiceDetailsAsync(Guid id, Employee loggedInEmployee, Guid tenantId, CancellationToken ct)
        {
            // 1. Structured Logging: Log entry with context
            _logger.LogInfo("Fetching Purchase Invoice details. InvoiceId: {InvoiceId}, TenantId: {TenantId}", id, tenantId);

            try
            {
                // Check if the employee has the necessary permissions
                var viewAllPermissionTask = HasPermissionAsync(PermissionsMaster.ViewAllPurchaseInvoice, loggedInEmployee.Id);
                var viewSelfPermissionTask = HasPermissionAsync(PermissionsMaster.ViewSelfPurchaseInvoice, loggedInEmployee.Id);

                var viewAllPermission = await viewAllPermissionTask;
                var viewSelfPermission = await viewSelfPermissionTask;

                if (!viewAllPermission && !viewSelfPermission)
                {
                    _logger.LogWarning("Access Denied: {EmployeeId} do not have permission to view purchase invoice details", loggedInEmployee.Id);
                    return ApiResponse<PurchaseInvoiceDetailsVM>.ErrorResponse("You do not have permission to view purchase invoice details.", "You do not have permission to view purchase invoice details.", 403);
                }

                await using var context = await _dbContextFactory.CreateDbContextAsync(ct);

                // 2. Performance: Use AsNoTracking for read-only queries. 
                // Use AsSplitQuery to avoid Cartesian explosion on multiple Includes.
                var purchaseInvoice = await context.PurchaseInvoiceDetails
                    .AsNoTracking()
                    .AsSplitQuery()
                    .Include(pid => pid.Organization)
                    .Include(pid => pid.Supplier)
                    .Include(pid => pid.Status)
                    .Include(pid => pid.CreatedBy).ThenInclude(e => e!.JobRole)
                    .Include(pid => pid.UpdatedBy).ThenInclude(e => e!.JobRole)
                    .Where(pid => pid.Id == id && pid.TenantId == tenantId)
                    .FirstOrDefaultAsync(ct);

                // 3. Validation: Handle Not Found immediately
                if (purchaseInvoice == null)
                {
                    _logger.LogWarning("Purchase Invoice not found or inactive. InvoiceId: {InvoiceId}", id);
                    return ApiResponse<PurchaseInvoiceDetailsVM>.ErrorResponse("Purchase invoice not found", "The specified purchase invoice does not exist or has been deleted.", 404);
                }

                // 4. Parallel Execution: Fetch Project details efficiently
                // Note: Assuming these methods return null if not found, rather than throwing.
                var infraProjectTask = LoadInfraProjectAsync(purchaseInvoice.ProjectId, tenantId);
                var serviceProjectTask = LoadServiceProjectAsync(purchaseInvoice.ProjectId, tenantId);

                await Task.WhenAll(infraProjectTask, serviceProjectTask);

                // Safely retrieve results without blocking .Result
                var project = await infraProjectTask ?? await serviceProjectTask;

                if (project == null)
                {
                    _logger.LogWarning("Data Inconsistency: Project not found for InvoiceId: {InvoiceId}, ProjectId: {ProjectId}", id, purchaseInvoice.ProjectId);
                    return ApiResponse<PurchaseInvoiceDetailsVM>.ErrorResponse("Project not found", "The project associated with this invoice could not be found.", 404);
                }

                // 5. Optimized Attachment Fetching
                var attachments = await context.PurchaseInvoiceAttachments
                    .AsNoTracking()
                    .Include(pia => pia.Document)
                    .Include(pia => pia.InvoiceAttachmentType)
                    .Where(pia =>
                        pia.PurchaseInvoiceId == id &&
                        pia.TenantId == tenantId &&
                        pia.Document != null &&
                        pia.InvoiceAttachmentType != null)
                    .ToListAsync(ct);

                // 6. Mapping & Transformation
                var response = _mapper.Map<PurchaseInvoiceDetailsVM>(purchaseInvoice);
                response.Project = project;

                if (attachments.Count > 0)
                {
                    response.Attachments = attachments.Select(a =>
                    {
                        var result = _mapper.Map<PurchaseInvoiceAttachmentVM>(a);

                        // Ensure S3 Key exists before generating URL to prevent SDK errors
                        if (a.Document != null)
                        {
                            result.PreSignedUrl = _s3Service.GeneratePreSignedUrl(a.Document.S3Key);

                            // Fallback logic for thumbnail
                            var thumbKey = !string.IsNullOrEmpty(a.Document.ThumbS3Key)
                                ? a.Document.ThumbS3Key
                                : a.Document.S3Key;

                            result.ThumbPreSignedUrl = _s3Service.GeneratePreSignedUrl(thumbKey);
                        }
                        return result;
                    }).ToList();
                }
                else
                {
                    response.Attachments = new List<PurchaseInvoiceAttachmentVM>();
                }

                _logger.LogInfo("Successfully fetched Purchase Invoice details. InvoiceId: {InvoiceId}", id);

                return ApiResponse<PurchaseInvoiceDetailsVM>.SuccessResponse(response, "Purchase invoice details fetched successfully.", 200);
            }
            catch (OperationCanceledException)
            {
                // Handle request cancellation (e.g., user navigates away)
                _logger.LogWarning("Request was cancelled by the user. InvoiceId: {InvoiceId}", id);
                return ApiResponse<PurchaseInvoiceDetailsVM>.ErrorResponse("Request Cancelled", "The operation was cancelled.", 499);
            }
            catch (Exception ex)
            {
                // 7. Global Error Handling
                _logger.LogError(ex, "An unhandled exception occurred while fetching Purchase Invoice. InvoiceId: {InvoiceId}", id);
                return ApiResponse<PurchaseInvoiceDetailsVM>.ErrorResponse("Internal Server Error", "An unexpected error occurred while processing your request. Please contact support.",
                    500);
            }
        }

        /// <summary>
        /// Creates a new Purchase Invoice with validation, S3 file uploads, and transactional database storage.
        /// </summary>
        /// <param name="model">The invoice data transfer object.</param>
        /// <param name="loggedInEmployee">The current user context.</param>
        /// <param name="tenantId">The tenant identifier.</param>
        /// <param name="ct">Cancellation token.</param>
        /// <returns>The created invoice view model wrapped in an API response.</returns>
        public async Task<ApiResponse<PurchaseInvoiceListVM>> CreatePurchaseInvoiceAsync(PurchaseInvoiceDto model, Employee loggedInEmployee, Guid tenantId, CancellationToken ct = default)
        {
            // 1. INPUT VALIDATION
            if (model == null) throw new ArgumentNullException(nameof(model));

            // Scoped variables to hold validation results to avoid fetching them again later
            BasicProjectVM? projectVm = null;
            Organization? organization = null;
            Organization? supplier = null;
            PurchaseInvoiceStatus? status = null;

            try
            {
                // Check permissions
                var manageInvoicesPermission = await HasPermissionAsync(PermissionsMaster.ManagePurchaseInvoice, loggedInEmployee.Id);
                if (!manageInvoicesPermission)
                {
                    _logger.LogWarning("Access Denied: {EmployeeId} do not have permission to create a purchase invoice", loggedInEmployee.Id);
                    return ApiResponse<PurchaseInvoiceListVM>.ErrorResponse("Access Denied", "You do not have permission to create a purchase invoice.", 403);
                }
                _logger.LogInfo("Initiating Purchase Invoice creation for ProjectId: {ProjectId}, TenantId: {TenantId}", model.ProjectId, tenantId);

                // 2. DATA VALIDATION (Fail-Fast Strategy)
                // We use a single Context instance here for read-only validation. 
                // It is more efficient than opening 5 parallel connections.
                await using var readContext = await _dbContextFactory.CreateDbContextAsync(ct);

                // A. Validate Project (Check Infra, if null check Service)
                // Optimized: Only fetch what is needed using Select first
                var infraProject = await readContext.Projects
                    .AsNoTracking()
                    .Where(p => p.Id == model.ProjectId && p.TenantId == tenantId)
                    .Select(p => _mapper.Map<BasicProjectVM>(p))
                    .FirstOrDefaultAsync(ct);

                if (infraProject == null)
                {
                    var serviceProject = await readContext.ServiceProjects
                        .AsNoTracking()
                        .Where(sp => sp.Id == model.ProjectId && sp.IsActive && sp.TenantId == tenantId)
                        .Select(p => _mapper.Map<BasicProjectVM>(p))
                        .FirstOrDefaultAsync(ct);

                    if (serviceProject == null)
                    {
                        _logger.LogWarning("CreatePurchaseInvoice failed: Project {ProjectId} not found for Tenant {TenantId}", model.ProjectId, tenantId);
                        return ApiResponse<PurchaseInvoiceListVM>.ErrorResponse("Project not found", "The specified project does not exist.", 404);
                    }
                    projectVm = serviceProject;
                }
                else
                {
                    projectVm = infraProject;
                }

                // B. Validate Organization
                organization = await readContext.Organizations
                    .AsNoTracking()
                    .FirstOrDefaultAsync(o => o.Id == model.OrganizationId && o.IsActive, ct);

                if (organization == null)
                {
                    _logger.LogWarning("CreatePurchaseInvoice failed: Organization {OrganizationId} not found.", model.OrganizationId);
                    return ApiResponse<PurchaseInvoiceListVM>.ErrorResponse("Organization not found", "The selected organization is invalid.", 404);
                }

                // C. Validate Supplier
                supplier = await readContext.Organizations
                    .AsNoTracking()
                    .FirstOrDefaultAsync(o => o.Id == model.SupplierId && o.IsActive, ct);

                if (supplier == null)
                {
                    _logger.LogWarning("CreatePurchaseInvoice failed: Supplier {SupplierId} not found.", model.SupplierId);
                    return ApiResponse<PurchaseInvoiceListVM>.ErrorResponse("Supplier not found", "The selected supplier is invalid.", 404);
                }

                // D. Validate Status
                status = await readContext.PurchaseInvoiceStatus
                    .AsNoTracking()
                    .FirstOrDefaultAsync(s => s.Id == DraftInvoiceStatusId, ct);

                if (status == null)
                {
                    _logger.LogWarning("CreatePurchaseInvoice critical: Default 'Draft' status ID {StatusId} is missing from DB.", DraftInvoiceStatusId);
                    return ApiResponse<PurchaseInvoiceListVM>.ErrorResponse("System Error", "Default invoice status configuration is missing.", 500);
                }

                // 3. PREPARE S3 UPLOADS (Optimistic Upload Pattern)
                // We upload files BEFORE opening the DB transaction to prevent locking the DB during slow network I/O.
                // If DB save fails, we will trigger a cleanup in the catch block.
                var uploadedS3Keys = new List<string>(); // Keep track for rollback
                var preparedDocuments = new List<Document>();
                var preparedAttachments = new List<PurchaseInvoiceAttachment>();

                // Generate Invoice ID early for S3 folder structure
                var newInvoiceId = Guid.NewGuid();

                if (model.Attachments.Any())
                {
                    var batchId = Guid.NewGuid();

                    // Fetch Attachment Types
                    var typeIds = model.Attachments.Select(a => a.InvoiceAttachmentTypeId).ToList();
                    var types = await readContext.InvoiceAttachmentTypes.Where(iat => typeIds.Contains(iat.Id)).ToListAsync(ct);

                    foreach (var attachment in model.Attachments)
                    {
                        // Validate Type
                        if (!types.Any(t => t.Id == attachment.InvoiceAttachmentTypeId))
                        {
                            _logger.LogWarning("CreatePurchaseInvoice failed: Attachment type {InvoiceAttachmentTypeId} is invalid.", attachment.InvoiceAttachmentTypeId);
                            return ApiResponse<PurchaseInvoiceListVM>.ErrorResponse("Invalid Attachment", $"Attachment type {attachment.InvoiceAttachmentTypeId} is invalid.", 400);
                        }

                        // Validate Base64
                        var base64Data = attachment.Base64Data?.Split(',').LastOrDefault();
                        if (string.IsNullOrWhiteSpace(base64Data))
                        {
                            _logger.LogWarning("CreatePurchaseInvoice failed: Attachment {FileName} contains no data.", attachment.FileName ?? "<unnamed>");
                            return ApiResponse<PurchaseInvoiceListVM>.ErrorResponse("Invalid Attachment", $"Attachment {attachment.FileName} contains no data.", 400);
                        }

                        // Process Metadata
                        var fileType = _s3Service.GetContentTypeFromBase64(base64Data);
                        // Use default extension if fileType extraction fails, prevents crashing
                        var safeFileType = string.IsNullOrEmpty(fileType) ? "application/octet-stream" : fileType;
                        var fileName = attachment.FileName ?? _s3Service.GenerateFileName(safeFileType, tenantId, "invoice");
                        var objectKey = $"tenant-{tenantId}/PurchaseInvoice/{newInvoiceId}/{fileName}";

                        // Perform Upload
                        await _s3Service.UploadFileAsync(base64Data, safeFileType, objectKey);
                        uploadedS3Keys.Add(objectKey); // Track for rollback

                        // Prepare Entities
                        var documentId = Guid.NewGuid();
                        preparedDocuments.Add(new Document
                        {
                            Id = documentId,
                            BatchId = batchId,
                            UploadedById = loggedInEmployee.Id,
                            FileName = fileName,
                            ContentType = attachment.ContentType ?? safeFileType,
                            S3Key = objectKey,
                            FileSize = attachment.FileSize,
                            UploadedAt = DateTime.UtcNow,
                            TenantId = tenantId
                        });

                        preparedAttachments.Add(new PurchaseInvoiceAttachment
                        {
                            Id = Guid.NewGuid(),
                            InvoiceAttachmentTypeId = attachment.InvoiceAttachmentTypeId,
                            PurchaseInvoiceId = newInvoiceId,
                            DocumentId = documentId,
                            UploadedAt = DateTime.UtcNow,
                            UploadedById = loggedInEmployee.Id,
                            TenantId = tenantId
                        });
                    }
                }

                // 4. TRANSACTIONAL PERSISTENCE
                await using var writeContext = await _dbContextFactory.CreateDbContextAsync(ct);

                // Use ExecutionStrategy for transient failure resiliency (e.g. cloud DB hiccups)
                var strategy = writeContext.Database.CreateExecutionStrategy();

                return await strategy.ExecuteAsync(async () =>
                {
                    await using var transaction = await writeContext.Database.BeginTransactionAsync(ct);
                    try
                    {
                        // A. UID Generation
                        // Note: In high concurrency, "Max + 1" can cause duplicates. 
                        // Ideally, lock the table or use a DB Sequence. 
                        string uIDPrefix = $"PUR/{DateTime.Now:MMyy}";

                        var lastInvoice = await writeContext.PurchaseInvoiceDetails
                            .Where(e => e.UIDPrefix == uIDPrefix && e.TenantId == tenantId) // Ensure Tenant Check
                            .OrderByDescending(e => e.UIDPostfix)
                            .Select(x => x.UIDPostfix) // Select only what we need
                            .FirstOrDefaultAsync(ct);

                        int uIDPostfix = (lastInvoice == 0 ? 0 : lastInvoice) + 1;

                        // B. Map & Add Invoice
                        var purchaseInvoice = _mapper.Map<PurchaseInvoiceDetails>(model);
                        purchaseInvoice.Id = newInvoiceId; // Set the ID we generated earlier
                        purchaseInvoice.UIDPrefix = uIDPrefix;
                        purchaseInvoice.UIDPostfix = uIDPostfix;
                        purchaseInvoice.StatusId = status.Id;
                        purchaseInvoice.CreatedAt = DateTime.UtcNow;
                        purchaseInvoice.CreatedById = loggedInEmployee.Id;
                        purchaseInvoice.IsActive = true;
                        purchaseInvoice.TenantId = tenantId;

                        writeContext.PurchaseInvoiceDetails.Add(purchaseInvoice);

                        // C. Add Documents if any
                        if (preparedDocuments.Any())
                        {
                            writeContext.Documents.AddRange(preparedDocuments);
                            writeContext.PurchaseInvoiceAttachments.AddRange(preparedAttachments);
                        }

                        await writeContext.SaveChangesAsync(ct);
                        await transaction.CommitAsync(ct);

                        _logger.LogInfo("Purchase Invoice created successfully. ID: {InvoiceId}, UID: {UID}", purchaseInvoice.Id, $"{uIDPrefix}-{uIDPostfix}");

                        // D. Prepare Response
                        var response = _mapper.Map<PurchaseInvoiceListVM>(purchaseInvoice);
                        response.Status = status;
                        response.Project = projectVm;
                        response.Supplier = _mapper.Map<BasicOrganizationVm>(supplier);

                        return ApiResponse<PurchaseInvoiceListVM>.SuccessResponse(response, "Purchase invoice created successfully", 201);
                    }
                    catch (DbUpdateException ex)
                    {
                        await transaction.RollbackAsync(ct);

                        // 5. COMPENSATION (S3 Rollback)
                        // If DB failed, we must delete the orphaned files from S3 to save cost and storage.
                        if (uploadedS3Keys != null && uploadedS3Keys.Any())
                        {
                            _logger.LogInfo("Rolling back S3 uploads for failed invoice creation.");
                            // Fire and forget cleanup, or await if strict consistency is required
                            foreach (var key in uploadedS3Keys)
                            {
                                try { await _s3Service.DeleteFileAsync(key); }
                                catch (Exception s3Ex) { _logger.LogError(s3Ex, "Failed to cleanup S3 file {Key} during rollback", key); }
                            }
                        }
                        _logger.LogError(ex, "Failed to create purchase invoice for Tenant {TenantId}. Error: {Message}", tenantId, ex.Message);

                        return ApiResponse<PurchaseInvoiceListVM>.ErrorResponse("Creation Failed", "An unexpected error occurred while processing your request.", 500);
                    }
                });
            }
            catch (Exception ex)
            {
                _logger.LogError(ex, "Failed to create purchase invoice for Tenant {TenantId}. Error: {Message}", tenantId, ex.Message);
                return ApiResponse<PurchaseInvoiceListVM>.ErrorResponse("Creation Failed", "An unexpected error occurred while processing your request.", 500);
            }
        }

        public async Task<ApiResponse<object>> UpdatePurchaseInvoiceAsync(Guid id, PurchaseInvoiceDetails purchaseInvoice, PurchaseInvoiceDto model, Employee loggedInEmployee, Guid tenantId, CancellationToken ct)
        {
            // Validate input arguments and log warnings for invalid cases.
            if (id == Guid.Empty)
            {
                _logger.LogWarning("UpdatePurchaseInvoiceAsync called with empty invoice Id. TenantId: {TenantId}, EmployeeId: {EmployeeId}", tenantId, loggedInEmployee.Id);

                return ApiResponse<object>.ErrorResponse(
                    "Invalid invoice identifier",
                    "The purchase invoice identifier cannot be empty.",
                    400);
            }

            if (tenantId == Guid.Empty)
            {
                _logger.LogWarning("UpdatePurchaseInvoiceAsync called with empty tenant Id. InvoiceId: {InvoiceId}, EmployeeId: {EmployeeId}", id, loggedInEmployee.Id);

                return ApiResponse<object>.ErrorResponse(
                    "Invalid tenant identifier",
                    "The tenant identifier cannot be empty.",
                    400);
            }
            // Check if user has permission
            var manageInvoicesPermission = await HasPermissionAsync(PermissionsMaster.ManagePurchaseInvoice, loggedInEmployee.Id);
            if (!manageInvoicesPermission)
            {
                _logger.LogWarning("Access Denied: {EmployeeId} do not have permission to update a purchase invoice", loggedInEmployee.Id);
                return ApiResponse<object>.ErrorResponse("Access Denied", "You do not have permission to update a purchase invoice.", 403);
            }

            _logger.LogInfo("Starting UpdatePurchaseInvoiceAsync. InvoiceId: {InvoiceId}, TenantId: {TenantId}, EmployeeId: {EmployeeId}", id, tenantId, loggedInEmployee.Id);

            await using var context = await _dbContextFactory.CreateDbContextAsync(ct);
            await using var transaction = await context.Database.BeginTransactionAsync();
            try
            {
                // Scoped helper service for update logs.
                using var scope = _serviceScopeFactory.CreateScope();
                var updateLogHelper = scope.ServiceProvider.GetRequiredService<UtilityMongoDBHelper>();

                // 1. Validate existence of Project (Infra or Service).
                var infraProject = await context.Projects
                    .AsNoTracking()
                    .Where(p => p.Id == model.ProjectId && p.TenantId == tenantId)
                    .Select(p => _mapper.Map<BasicProjectVM>(p))
                    .FirstOrDefaultAsync(ct);

                BasicProjectVM? projectVm;
                if (infraProject == null)
                {
                    var serviceProject = await context.ServiceProjects
                        .AsNoTracking()
                        .Where(sp => sp.Id == model.ProjectId && sp.IsActive && sp.TenantId == tenantId)
                        .Select(p => _mapper.Map<BasicProjectVM>(p))
                        .FirstOrDefaultAsync(ct);

                    if (serviceProject == null)
                    {
                        _logger.LogWarning("UpdatePurchaseInvoiceAsync failed: Project {ProjectId} not found for Tenant {TenantId}", model.ProjectId, tenantId);

                        return ApiResponse<object>.ErrorResponse("Project not found", "The specified project does not exist.", 404);
                    }
                    projectVm = serviceProject;
                }
                else
                {
                    projectVm = infraProject;
                }

                // 2. Validate Organization.
                var organization = await context.Organizations
                    .AsNoTracking()
                    .FirstOrDefaultAsync(o => o.Id == model.OrganizationId && o.IsActive, ct);

                if (organization == null)
                {
                    _logger.LogWarning("UpdatePurchaseInvoiceAsync failed: Organization {OrganizationId} not found or inactive.", model.OrganizationId);

                    return ApiResponse<object>.ErrorResponse("Organization not found", "The selected organization is invalid or inactive.", 404);
                }

                // 3. Validate Supplier.
                var supplier = await context.Organizations
                    .AsNoTracking()
                    .FirstOrDefaultAsync(o => o.Id == model.SupplierId && o.IsActive, ct);

                if (supplier == null)
                {
                    _logger.LogWarning("UpdatePurchaseInvoiceAsync failed: Supplier {SupplierId} not found or inactive.", model.SupplierId);

                    return ApiResponse<object>.ErrorResponse("Supplier not found", "The selected supplier is invalid or inactive.", 404);
                }

                // 4. Validate PurchaseInvoiceStatus.
                var status = await context.PurchaseInvoiceStatus
                    .AsNoTracking()
                    .FirstOrDefaultAsync(s => s.Id == model.StatusId, ct);

                if (status == null)
                {
                    _logger.LogError(null, "UpdatePurchaseInvoiceAsync critical: Missing required purchase invoice status ID {StatusId}.", model.StatusId ?? Guid.Empty);

                    return ApiResponse<object>.ErrorResponse(
                        "System configuration error",
                        "Required purchase invoice status configuration is missing in the system.",
                        500);
                }

                // Save previous state for audit/logging.
                var existingEntityBson = updateLogHelper.EntityToBsonDocument(purchaseInvoice);

                // Map updated fields from DTO to entity.
                _mapper.Map(model, purchaseInvoice);
                purchaseInvoice.UpdatedAt = DateTime.UtcNow;
                purchaseInvoice.UpdatedById = loggedInEmployee.Id;

                context.PurchaseInvoiceDetails.Update(purchaseInvoice);
                await context.SaveChangesAsync(ct);

                // 5. Handle attachments update.
                var newAttachments = model.Attachments.Where(a => !a.DocumentId.HasValue && a.IsActive).ToList();
                var deleteAttachmentIds = model.Attachments
                    .Where(a => a.DocumentId.HasValue && !a.IsActive)
                    .Select(a => a.DocumentId!.Value)
                    .ToList();

                if (newAttachments.Any())
                {
                    var batchId = Guid.NewGuid();

                    // Validate attachment types.
                    var typeIds = newAttachments.Select(a => a.InvoiceAttachmentTypeId).Distinct().ToList();
                    var validTypes = await context.InvoiceAttachmentTypes
                                        .Where(iat => typeIds.Contains(iat.Id))
                                        .ToListAsync(ct);

                    var preparedDocuments = new List<Document>();
                    var preparedAttachments = new List<PurchaseInvoiceAttachment>();

                    // Process each new attachment.
                    foreach (var attachment in newAttachments)
                    {
                        var attachmentType = validTypes.FirstOrDefault(t => t.Id == attachment.InvoiceAttachmentTypeId);
                        if (attachmentType == null)
                        {
                            _logger.LogWarning("UpdatePurchaseInvoiceAsync failed: Invalid attachment type ID {AttachmentTypeId}.", attachment.InvoiceAttachmentTypeId);
                            return ApiResponse<object>.ErrorResponse("Invalid attachment types", $"One or more attachment types are invalid: {attachment.InvoiceAttachmentTypeId}", 400);
                        }
                        // Validate base64 data presence.
                        var base64Data = attachment.Base64Data?.Split(',').LastOrDefault();
                        if (string.IsNullOrWhiteSpace(base64Data))
                        {
                            _logger.LogWarning("UpdatePurchaseInvoiceAsync failed: Attachment '{FileName}' contains no data.", attachment.FileName ?? "<unnamed>");

                            return ApiResponse<object>.ErrorResponse("Invalid attachment", $"Attachment '{attachment.FileName ?? "<unnamed>"}' contains no valid data.", 400);
                        }

                        // Determine content type with fallback.
                        var fileType = _s3Service.GetContentTypeFromBase64(base64Data);
                        var safeFileType = string.IsNullOrEmpty(fileType) ? "application/octet-stream" : fileType;

                        var fileName = attachment.FileName ?? _s3Service.GenerateFileName(safeFileType, tenantId, "invoice");
                        var objectKey = $"tenant-{tenantId}/PurchaseInvoice/{id}/{fileName}";

                        // Upload file to S3 asynchronously.
                        await _s3Service.UploadFileAsync(base64Data, safeFileType, objectKey);

                        var documentId = Guid.NewGuid();

                        // Prepare Document entity.
                        preparedDocuments.Add(new Document
                        {
                            Id = documentId,
                            BatchId = batchId,
                            UploadedById = loggedInEmployee.Id,
                            FileName = fileName,
                            ContentType = attachment.ContentType ?? safeFileType,
                            S3Key = objectKey,
                            FileSize = attachment.FileSize,
                            UploadedAt = DateTime.UtcNow,
                            TenantId = tenantId,
                        });

                        // Prepare PurchaseInvoiceAttachment entity.
                        preparedAttachments.Add(new PurchaseInvoiceAttachment
                        {
                            Id = Guid.NewGuid(),
                            InvoiceAttachmentTypeId = attachmentType.Id,
                            PurchaseInvoiceId = id,
                            DocumentId = documentId,
                            UploadedAt = DateTime.UtcNow,
                            UploadedById = loggedInEmployee.Id,
                            TenantId = tenantId,
                        });
                    }

                    // Add batched uploaded documents and attachments.
                    context.Documents.AddRange(preparedDocuments);
                    context.PurchaseInvoiceAttachments.AddRange(preparedAttachments);
                    await context.SaveChangesAsync(ct);
                }

                // Delete attachments marked for removal.
                if (deleteAttachmentIds.Any())
                {
                    await DeleteAttachemnts(deleteAttachmentIds, ct);
                    _logger.LogInfo("Deleted {Count} attachments from PurchaseInvoiceId {InvoiceId} for TenantId {TenantId}", deleteAttachmentIds.Count, id, tenantId);
                }

                await transaction.CommitAsync();

                // Push audit log entry asynchronously for traceability.
                await updateLogHelper.PushToUpdateLogsAsync(
                    new UpdateLogsObject
                    {
                        EntityId = id.ToString(),
                        UpdatedById = loggedInEmployee.Id.ToString(),
                        OldObject = existingEntityBson,
                        UpdatedAt = DateTime.UtcNow
                    },
                    "PurchaseInvoiceModificationLog");

                _logger.LogInfo("Purchase invoice updated successfully. InvoiceId: {InvoiceId}, TenantId: {TenantId}, UpdatedById: {UserId}", id, tenantId, loggedInEmployee.Id);

                var response = _mapper.Map<PurchaseInvoiceListVM>(purchaseInvoice);
                response.Project = projectVm;

                return ApiResponse<object>.SuccessResponse(response, "Purchase invoice updated successfully.", 200);
            }
            catch (OperationCanceledException)
            {
                await transaction.RollbackAsync();
                _logger.LogError(null, "UpdatePurchaseInvoiceAsync operation cancelled. InvoiceId: {InvoiceId}, TenantId: {TenantId}", id, tenantId);

                return ApiResponse<object>.ErrorResponse("Request cancelled", "The update operation was cancelled by the client or the server.", 499);
            }
            catch (DbUpdateException ex)
            {
                await transaction.RollbackAsync();
                _logger.LogError(ex, "Unexpected error during update of purchase invoice. InvoiceId: {InvoiceId}, TenantId: {TenantId}, UserId: {UserId}", id, tenantId, loggedInEmployee.Id);

                return ApiResponse<object>.ErrorResponse("Update failed", "An unexpected error occurred while updating the purchase invoice. Please try again later.", 500);
            }
            catch (Exception ex)
            {
                _logger.LogError(ex, "Unexpected error during update of purchase invoice. InvoiceId: {InvoiceId}, TenantId: {TenantId}, UserId: {UserId}", id, tenantId, loggedInEmployee.Id);

                return ApiResponse<object>.ErrorResponse("Update failed", "An unexpected error occurred while updating the purchase invoice. Please try again later.", 500);
            }
        }

        public async Task<ApiResponse<object>> DeletePurchaseInvoice(Guid id, bool isActive, Employee loggedInEmployee, Guid tenantId, CancellationToken ct)
        {
            // Check if the employee has the necessary permissions
            var deletePermission = await HasPermissionAsync(PermissionsMaster.DeletePurchaseInvoice, loggedInEmployee.Id);
            if (!deletePermission)
            {
                _logger.LogWarning("DeletePurchaseInvoiceAsync failed: EmployeeId {EmployeeId} does not have permission.", loggedInEmployee.Id);
                return ApiResponse<object>.ErrorResponse("Permission denied", "You do not have permission to delete this invoice.", 403);
            }

            await using var context = await _dbContextFactory.CreateDbContextAsync(ct);

            var purchaseInvoice = await context.PurchaseInvoiceDetails.FirstOrDefaultAsync(x => x.Id == id && x.TenantId == tenantId, ct);
            if (purchaseInvoice == null)
            {
                _logger.LogWarning("DeletePurchaseInvoiceAsync failed: InvoiceId {InvoiceId} not found.", id);
                return ApiResponse<object>.ErrorResponse("Invoice not found", "The invoice with the specified ID was not found.", 404);
            }

            using var scope = _serviceScopeFactory.CreateScope();
            var updateLogHelper = scope.ServiceProvider.GetRequiredService<UtilityMongoDBHelper>();

            var existingEntityBson = updateLogHelper.EntityToBsonDocument(purchaseInvoice);

            purchaseInvoice.IsActive = isActive;

            await context.SaveChangesAsync(ct);

            await updateLogHelper.PushToUpdateLogsAsync(
                new UpdateLogsObject
                {
                    EntityId = id.ToString(),
                    UpdatedById = loggedInEmployee.Id.ToString(),
                    OldObject = existingEntityBson,
                    UpdatedAt = DateTime.UtcNow
                },
                "PurchaseInvoiceModificationLog");

            return ApiResponse<object>.SuccessResponse(new { }, "Invoice deleted successfully.", 200);
        }

        /// <summary>
        /// Soft-deletes or restores a Purchase Invoice by toggling its active flag,
        /// with permission checks, audit logging, and structured logging suitable
        /// for enterprise-grade observability.
        /// </summary>
        /// <param name="id">The Purchase Invoice identifier.</param>
        /// <param name="isActive">
        /// Indicates the new active state:
        /// false = mark as deleted/inactive (soft delete),
        /// true = restore/reactivate.
        /// </param>
        /// <param name="loggedInEmployee">The currently logged-in employee performing the operation.</param>
        /// <param name="tenantId">Tenant identifier to enforce multi-tenant isolation.</param>
        /// <param name="ct">Cancellation token for cooperative cancellation.</param>
        /// <returns>
        /// Standardized <see cref="ApiResponse{T}"/> with operation result or error details.
        /// </returns>
        public async Task<ApiResponse<object>> DeletePurchaseInvoiceAsync(Guid id, bool isActive, Employee loggedInEmployee, Guid tenantId, CancellationToken ct)
        {
            // Guard clause: validate invoice identifier.
            if (id == Guid.Empty)
            {
                _logger.LogWarning("DeletePurchaseInvoiceAsync called with empty InvoiceId. TenantId: {TenantId}, EmployeeId: {EmployeeId}", tenantId, loggedInEmployee.Id);
                return ApiResponse<object>.ErrorResponse("Invoice reference is required.", "DeletePurchaseInvoiceAsync received an empty invoice Id.", 400);
            }

            try
            {
                // Step 1: Permission check for the current employee.
                var hasDeletePermission = await HasPermissionAsync(PermissionsMaster.DeletePurchaseInvoice, loggedInEmployee.Id);

                if (!hasDeletePermission)
                {
                    _logger.LogWarning("DeletePurchaseInvoiceAsync permission denied. InvoiceId: {InvoiceId}, TenantId: {TenantId}, EmployeeId: {EmployeeId}",
                        id, tenantId, loggedInEmployee.Id);

                    return ApiResponse<object>.ErrorResponse("You do not have permission to modify this invoice.", "DeletePurchaseInvoiceAsync failed due to missing DeletePurchaseInvoice permission.",
                        403);
                }

                // Step 2: Create a short-lived DbContext for this operation.
                await using var context = await _dbContextFactory.CreateDbContextAsync(ct);

                // Step 3: Retrieve the invoice scoped to the current tenant.
                var purchaseInvoice = await context.PurchaseInvoiceDetails
                    .FirstOrDefaultAsync(x => x.Id == id && x.TenantId == tenantId, ct);

                if (purchaseInvoice == null)
                {
                    _logger.LogWarning(
                        "DeletePurchaseInvoiceAsync failed: Invoice not found. InvoiceId: {InvoiceId}, TenantId: {TenantId}, EmployeeId: {EmployeeId}",
                        id, tenantId, loggedInEmployee.Id);

                    return ApiResponse<object>.ErrorResponse("Invoice not found.", $"Purchase invoice not found for Id: {id}, TenantId: {tenantId}.", 404);
                }

                // Step 4: Create a scoped helper for MongoDB update logs/audit trail.
                using var scope = _serviceScopeFactory.CreateScope();
                var updateLogHelper = scope.ServiceProvider.GetRequiredService<UtilityMongoDBHelper>();

                // Capture the existing state for audit logging before modification.
                var existingEntityBson = updateLogHelper.EntityToBsonDocument(purchaseInvoice);

                // Step 5: Apply the soft-delete or restore operation.
                purchaseInvoice.IsActive = isActive;

                // Persist changes with cancellation support.
                var rowsAffected = await context.SaveChangesAsync(ct);

                if (rowsAffected <= 0)
                {
                    _logger.LogError(null, "DeletePurchaseInvoiceAsync failed to persist changes. InvoiceId: {InvoiceId}, TenantId: {TenantId}, EmployeeId: {EmployeeId}",
                        id, tenantId, loggedInEmployee.Id);
                    return ApiResponse<object>.ErrorResponse("Failed to update the invoice status.", "DeletePurchaseInvoiceAsync SaveChangesAsync returned 0 rows affected.", 500);
                }

                // Step 6: Push audit log to MongoDB (non-critical but important for traceability).
                await updateLogHelper.PushToUpdateLogsAsync(
                    new UpdateLogsObject
                    {
                        EntityId = id.ToString(),
                        UpdatedById = loggedInEmployee.Id.ToString(),
                        OldObject = existingEntityBson,
                        UpdatedAt = DateTime.UtcNow
                    },
                    "PurchaseInvoiceModificationLog");

                _logger.LogInfo("DeletePurchaseInvoiceAsync completed successfully. InvoiceId: {InvoiceId}, TenantId: {TenantId}, EmployeeId: {EmployeeId}, NewIsActive: {IsActive}",
                    id, tenantId, loggedInEmployee.Id, isActive);

                var action = isActive ? "restored" : "deleted";

                return ApiResponse<object>.SuccessResponse(new { InvoiceId = id, IsActive = isActive }, $"Invoice has been {action} successfully.", 200);
            }
            catch (OperationCanceledException)
            {
                // Explicit cancellation handling to avoid misclassification as an error.
                _logger.LogError(null, "DeletePurchaseInvoiceAsync operation was canceled. InvoiceId: {InvoiceId}, TenantId: {TenantId}, EmployeeId: {EmployeeId}",
                    id, tenantId, loggedInEmployee.Id);
                return ApiResponse<object>.ErrorResponse("The operation was canceled.", "DeletePurchaseInvoiceAsync was canceled by the caller.", 499);
            }
            catch (DbUpdateException dbEx)
            {
                // Database-related error with structured logging.
                _logger.LogError(dbEx, "Database update error in DeletePurchaseInvoiceAsync. InvoiceId: {InvoiceId}, TenantId: {TenantId}, EmployeeId: {EmployeeId}",
                    id, tenantId, loggedInEmployee.Id);
                return ApiResponse<object>.ErrorResponse("An error occurred while updating the invoice.", "Database update exception occurred in DeletePurchaseInvoiceAsync.", 500);
            }
            catch (Exception ex)
            {
                // Catch-all for any unexpected failures.
                _logger.LogError(ex, "Unexpected error in DeletePurchaseInvoiceAsync. InvoiceId: {InvoiceId}, TenantId: {TenantId}, EmployeeId: {EmployeeId}",
                    id, tenantId, loggedInEmployee.Id);
                return ApiResponse<object>.ErrorResponse("An unexpected error occurred while updating the invoice status.", "Unhandled exception in DeletePurchaseInvoiceAsync.", 500);
            }
        }


        #endregion

        #region =================================================================== Delivery Challan Functions ===================================================================

        public async Task<ApiResponse<List<DeliveryChallanVM>>> GetDeliveryChallansAsync(Guid purchaseInvoiceId, Employee loggedInEmployee, Guid tenantId, CancellationToken ct)
        {
            // 1. Setup Context
            // Using a factory ensures a clean context for this specific unit of work.
            await using var context = await _dbContextFactory.CreateDbContextAsync(ct);

            try
            {
                // Check if the employee has the necessary permissions
                var viewAllPermissionTask = HasPermissionAsync(PermissionsMaster.ViewAllPurchaseInvoice, loggedInEmployee.Id);
                var viewSelfPermissionTask = HasPermissionAsync(PermissionsMaster.ViewSelfPurchaseInvoice, loggedInEmployee.Id);

                var viewAllPermission = await viewAllPermissionTask;
                var viewSelfPermission = await viewSelfPermissionTask;

                if (!viewAllPermission && !viewSelfPermission)
                {
                    _logger.LogWarning("Access Denied: {EmployeeId} do not have permission to view delivery challan list", loggedInEmployee.Id);
                    return ApiResponse<List<DeliveryChallanVM>>.ErrorResponse("You do not have permission to view delivery challan list.", "You do not have permission to view delivery challan list.", 403);
                }

                _logger.LogInfo("GetDeliveryChallans: Fetching challans. InvoiceId: {InvoiceId}, Tenant: {TenantId}", purchaseInvoiceId, tenantId);

                // 2. Optimized Validation
                // Use AnyAsync() instead of FirstOrDefaultAsync(). 
                // We only need to know if it *exists*, we don't need to load the data into memory.
                var isInvoiceValid = await context.PurchaseInvoiceDetails
                    .AsNoTracking()
                    .AnyAsync(pid => pid.Id == purchaseInvoiceId && pid.TenantId == tenantId, ct);

                if (!isInvoiceValid)
                {
                    _logger.LogWarning("GetDeliveryChallans: Invoice not found. InvoiceId: {InvoiceId}", purchaseInvoiceId);
                    return ApiResponse<List<DeliveryChallanVM>>.ErrorResponse("Invalid Purchase Invoice", "The specified purchase invoice does not exist.", 404);
                }

                // 3. Data Retrieval
                // Fetch only valid records with necessary related data.
                var deliveryChallanEntities = await context.DeliveryChallanDetails
                    .AsNoTracking()
                    .Include(dc => dc.PurchaseInvoice)
                    .Include(dc => dc.Attachment).ThenInclude(pia => pia!.Document)
                    .Where(dc => dc.PurchaseInvoiceId == purchaseInvoiceId
                                 && dc.TenantId == tenantId
                                 && dc.Attachment != null
                                 && dc.Attachment.Document != null) // Ensure strict data integrity
                    .ToListAsync(ct);

                // 4. Early Exit for Empty Lists
                // Returns an empty list with 200 OK immediately, avoiding unnecessary mapping/looping.
                if (!deliveryChallanEntities.Any())
                {
                    _logger.LogInfo("GetDeliveryChallans: No challans found for InvoiceId: {InvoiceId}", purchaseInvoiceId);
                    return ApiResponse<List<DeliveryChallanVM>>.SuccessResponse(new List<DeliveryChallanVM>(), "No delivery challans found.", 200);
                }

                // 5. Mapping and Transformation
                // We map the entities to View Models first, then apply business logic (S3 URLs).
                // Using Map<List<T>> is generally more efficient than mapping inside a Select loop for complex objects.

                // Enhance VMs with Signed URLs
                // We iterate through the already-mapped list to populate non-database fields.
                // Zip or standard for-loop could be used, but since we mapped a list, we need to match them up.
                // Note: Automapper preserves order, so index matching works, but iterating the Source Entity to populate the Dest VM is safer.

                var responseList = deliveryChallanEntities.Select(dc =>
                {
                    var result = _mapper.Map<DeliveryChallanVM>(dc);
                    if (dc.Attachment?.Document != null)
                    {
                        result.Attachment!.PreSignedUrl = _s3Service.GeneratePreSignedUrl(dc.Attachment.Document.S3Key);

                        // Fallback logic for thumbnail
                        var thumbKey = !string.IsNullOrEmpty(dc.Attachment.Document.ThumbS3Key)
                            ? dc.Attachment.Document.ThumbS3Key
                            : dc.Attachment.Document.S3Key;

                        result.Attachment.ThumbPreSignedUrl = _s3Service.GeneratePreSignedUrl(thumbKey);
                    }
                    return result;
                }).ToList();

                _logger.LogInfo("GetDeliveryChallans: Successfully returned {Count} items.", responseList.Count);

                return ApiResponse<List<DeliveryChallanVM>>.SuccessResponse(responseList, "List of delivery challans fetched successfully.", 200);
            }
            catch (Exception ex)
            {
                _logger.LogError(ex, "GetDeliveryChallans: An error occurred. InvoiceId: {InvoiceId}", purchaseInvoiceId);
                return ApiResponse<List<DeliveryChallanVM>>.ErrorResponse("Internal Server Error", "An unexpected error occurred while fetching delivery challans.", 500);
            }
        }
        public async Task<ApiResponse<DeliveryChallanVM>> AddDeliveryChallanAsync(DeliveryChallanDto model, Employee loggedInEmployee, Guid tenantId, CancellationToken ct)
        {
            // 1. Input Validation - Fail Fast
            // Validate inputs before engaging expensive resources (DB/S3).
            if (model == null) throw new ArgumentNullException(nameof(model));

            // 2. Security Check
            var addDeliveryChallanPermission = await HasPermissionAsync(PermissionsMaster.AddDeliveryChallan, loggedInEmployee.Id);
            if (!addDeliveryChallanPermission)
            {
                _logger.LogWarning("Access Denied: {EmployeeId} do not have permission to add delivery challan", loggedInEmployee.Id);
                return ApiResponse<DeliveryChallanVM>.ErrorResponse("You do not have permission to add delivery challan.", "You do not have permission to add delivery challan.", 403);
            }

            // Extract Base64 Data safely
            var base64Data = model.Attachment.Base64Data?.Split(',').LastOrDefault();
            if (string.IsNullOrWhiteSpace(base64Data))
            {
                _logger.LogWarning("AddDeliveryChallan: Validation Failed - Attachment is empty. Tenant: {TenantId}, Invoice: {InvoiceId}", tenantId, model.PurchaseInvoiceId);
                return ApiResponse<DeliveryChallanVM>.ErrorResponse("Invalid Attachment", "The uploaded attachment contains no data.", 400);
            }

            // Prepare S3 Metadata
            var fileType = _s3Service.GetContentTypeFromBase64(base64Data);
            var safeContentType = string.IsNullOrEmpty(fileType) ? "application/octet-stream" : fileType;
            // Use the sanitized file name or generate a new one to prevent path traversal or collision
            var fileName = !string.IsNullOrWhiteSpace(model.Attachment.FileName)
                ? model.Attachment.FileName
                : _s3Service.GenerateFileName(safeContentType, tenantId, "invoice");

            var objectKey = $"tenant-{tenantId}/PurchaseInvoice/{model.PurchaseInvoiceId}/{fileName}";

            // Generate new IDs upfront to maintain referential integrity in code
            var documentId = Guid.NewGuid();
            var attachmentId = Guid.NewGuid();
            var deliveryChallanId = Guid.NewGuid();

            // 3. Database Read Operations (Scoped Context)
            // We use a factory to create a short-lived context. 
            await using var context = await _dbContextFactory.CreateDbContextAsync(ct);

            // Fetch Purchase Invoice - Use AsNoTracking for performance since we aren't modifying it here.
            // Note: We project only what we need or map later to avoid EF translation issues with complex Mappers.
            var purchaseInvoiceEntity = await context.PurchaseInvoiceDetails
                .AsNoTracking()
                .FirstOrDefaultAsync(pid => pid.Id == model.PurchaseInvoiceId && pid.TenantId == tenantId, ct);

            if (purchaseInvoiceEntity == null)
            {
                _logger.LogWarning("AddDeliveryChallan: Purchase Invoice not found. Id: {InvoiceId}, Tenant: {TenantId}", model.PurchaseInvoiceId, tenantId);
                return ApiResponse<DeliveryChallanVM>.ErrorResponse("Not Found", "The specified Purchase Invoice does not exist.", 404);
            }

            // Validate Attachment Type
            var invoiceAttachmentType = await context.InvoiceAttachmentTypes
                .AsNoTracking()
                .FirstOrDefaultAsync(iat => iat.Id == DeliveryChallanTypeId, ct);

            if (invoiceAttachmentType == null)
            {
                _logger.LogError(null, "AddDeliveryChallan: Configuration Error - InvoiceAttachmentType {TypeId} missing.", DeliveryChallanTypeId);
                return ApiResponse<DeliveryChallanVM>.ErrorResponse("Configuration Error", "System configuration for Delivery Challan is missing.", 500);
            }

            // 4. External Service Call (S3 Upload)
            // We upload BEFORE the DB transaction. If this fails, we return error. 
            // If DB fails later, we must compensate (delete this file).
            try
            {
                await _s3Service.UploadFileAsync(base64Data, safeContentType, objectKey);
            }
            catch (Exception ex)
            {
                _logger.LogError(ex, "AddDeliveryChallan: S3 Upload failed. Key: {ObjectKey}", objectKey);
                return ApiResponse<DeliveryChallanVM>.ErrorResponse("Upload Failed", "Failed to upload the attachment to storage.", 502);
            }

            // 5. Transactional Write Operations
            // Begin transaction for data consistency across multiple tables.
            await using var transaction = await context.Database.BeginTransactionAsync(ct);

            try
            {
                var now = DateTime.UtcNow;

                // Entity 1: Document (Metadata)
                var document = new Document
                {
                    Id = documentId,
                    BatchId = Guid.NewGuid(), // Assuming single batch for this operation
                    UploadedById = loggedInEmployee.Id,
                    FileName = fileName,
                    ContentType = model.Attachment.ContentType ?? safeContentType,
                    S3Key = objectKey,
                    FileSize = model.Attachment.FileSize, // Ensure this is calculated correctly in DTO or here
                    UploadedAt = now,
                    TenantId = tenantId
                };

                // Entity 2: PurchaseInvoiceAttachment (Link)
                var newAttachment = new PurchaseInvoiceAttachment
                {
                    Id = attachmentId,
                    InvoiceAttachmentTypeId = DeliveryChallanTypeId,
                    PurchaseInvoiceId = model.PurchaseInvoiceId,
                    DocumentId = documentId,
                    UploadedAt = now,
                    UploadedById = loggedInEmployee.Id,
                    TenantId = tenantId
                };

                // Entity 3: DeliveryChallanDetails (Domain Data)
                var deliveryChallan = _mapper.Map<DeliveryChallanDetails>(model);
                deliveryChallan.Id = deliveryChallanId;
                deliveryChallan.AttachmentId = attachmentId;
                deliveryChallan.CreatedAt = now;
                deliveryChallan.CreatedById = loggedInEmployee.Id;
                deliveryChallan.TenantId = tenantId;

                // Batch Add
                context.Documents.Add(document);
                context.PurchaseInvoiceAttachments.Add(newAttachment);
                context.DeliveryChallanDetails.Add(deliveryChallan);

                // Execute DB changes - One round trip
                await context.SaveChangesAsync(ct);
                await transaction.CommitAsync(ct);

                _logger.LogInfo("AddDeliveryChallan: Success. ChallanId: {ChallanId}, Tenant: {TenantId}", deliveryChallanId, tenantId);

                // 6. Response Preparation
                // Map response objects. Ensure the VM matches the generic return type.
                var response = _mapper.Map<DeliveryChallanVM>(deliveryChallan);

                // Manual mapping for complex nested objects if Automapper config is not set for deep linking
                response.PurchaseInvoice = _mapper.Map<BasicPurchaseInvoiceVM>(purchaseInvoiceEntity);
                response.CreatedBy = _mapper.Map<BasicEmployeeVM>(loggedInEmployee);

                response.Attachment = new PurchaseInvoiceAttachmentVM
                {
                    DocumentId = document.Id,
                    InvoiceAttachmentType = invoiceAttachmentType,
                    FileName = document.FileName,
                    ContentType = document.ContentType,
                    // Generate URLs only when needed to keep response lightweight, or if they expire
                    PreSignedUrl = _s3Service.GeneratePreSignedUrl(objectKey),
                    ThumbPreSignedUrl = _s3Service.GeneratePreSignedUrl(objectKey)
                };

                return ApiResponse<DeliveryChallanVM>.SuccessResponse(response, "Delivery Challan added successfully.", 201); // 201 Created
            }
            catch (Exception ex)
            {
                // 7. Rollback & Compensation
                await transaction.RollbackAsync(ct);

                _logger.LogError(ex, "AddDeliveryChallan: Database transaction failed. Rolling back. Tenant: {TenantId}", tenantId);

                // Compensating Action: Delete the file from S3 since DB insert failed.
                // We run this in a fire-and-forget or background manner, or await it carefully so it doesn't hide the original exception.
                try
                {
                    _logger.LogInfo("AddDeliveryChallan: Attempting to delete orphaned S3 file: {ObjectKey}", objectKey);
                    await _s3Service.DeleteFileAsync(objectKey);
                }
                catch (Exception s3Ex)
                {
                    // Just log this, don't throw, so we still return the original DB error to the user
                    _logger.LogError(s3Ex, "AddDeliveryChallan: Failed to clean up orphaned S3 file: {ObjectKey}", objectKey);
                }

                return ApiResponse<DeliveryChallanVM>.ErrorResponse("Processing Error", "An error occurred while saving the delivery challan.", 500);
            }
        }
        #endregion

        #region =================================================================== Purchase Invoice History Functions ===================================================================

        /// <summary>
        /// Retrieves the full payment history for a given Purchase Invoice,
        /// including related adjustment heads and creator information,
        /// with multi-tenant safety and structured logging.
        /// </summary>
        /// <param name="purchaseInvoiceId">Identifier of the purchase invoice.</param>
        /// <param name="loggedInEmployee">The employee requesting the history.</param>
        /// <param name="tenantId">Current tenant identifier for multi-tenancy boundary.</param>
        /// <param name="ct">Cancellation token for cooperative cancellation.</param>
        /// <returns>
        /// Standardized <see cref="ApiResponse{T}"/> containing the list of payment history
        /// view models or a detailed error response.
        /// </returns>
        public async Task<ApiResponse<object>> GetPurchaseInvoiceHistoryListAsync(Guid purchaseInvoiceId, Employee loggedInEmployee, Guid tenantId, CancellationToken ct)
        {
            // Guard clauses to fail fast on obviously invalid parameters.
            if (purchaseInvoiceId == Guid.Empty)
            {
                _logger.LogWarning("GetPurchaseInvoiceHistoryListAsync called with empty PurchaseInvoiceId. TenantId: {TenantId}, EmployeeId: {EmployeeId}", tenantId, loggedInEmployee.Id);

                return ApiResponse<object>.ErrorResponse("Purchase invoice reference is required.", "PurchaseInvoiceId is empty.", 400);
            }

            try
            {
                // Check if the employee has the necessary permissions
                var viewAllPermissionTask = HasPermissionAsync(PermissionsMaster.ViewAllPurchaseInvoice, loggedInEmployee.Id);
                var viewSelfPermissionTask = HasPermissionAsync(PermissionsMaster.ViewSelfPurchaseInvoice, loggedInEmployee.Id);

                var viewAllPermission = await viewAllPermissionTask;
                var viewSelfPermission = await viewSelfPermissionTask;

                if (!viewAllPermission && !viewSelfPermission)
                {
                    _logger.LogWarning("Access Denied: {EmployeeId} do not have permission to view purchase invoice payment history list", loggedInEmployee.Id);
                    return ApiResponse<object>.ErrorResponse("You do not have permission to view purchase invoice payment history list.", "You do not have permission to view purchase invoice payment history list.", 403);
                }

                // Create a short-lived DbContext instance via factory for this operation.
                await using var context = await _dbContextFactory.CreateDbContextAsync(ct);

                // Step 1: Ensure the invoice exists for this tenant.
                var purchaseInvoice = await context.PurchaseInvoiceDetails
                    .AsNoTracking()
                    .FirstOrDefaultAsync(
                        pi => pi.Id == purchaseInvoiceId && pi.TenantId == tenantId,
                        ct);

                if (purchaseInvoice == null)
                {
                    _logger.LogWarning("Purchase Invoice not found. InvoiceId: {InvoiceId}, TenantId: {TenantId}, EmployeeId: {EmployeeId}", purchaseInvoiceId, tenantId, loggedInEmployee.Id);

                    return ApiResponse<object>.ErrorResponse("Purchase invoice not found.", $"Purchase invoice not found for InvoiceId: {purchaseInvoiceId}, TenantId: {tenantId}.", 404);
                }

                // Step 2: Query payment history with necessary related data eagerly loaded.
                var paymentHistoryQuery = context.PurchaseInvoicePayments
                    .Include(pip => pip.PaymentAdjustmentHead)
                    .Include(pip => pip.CreatedBy) // Include creator
                        .ThenInclude(e => e!.JobRole) // Include creator's job role
                    .Where(pip => pip.InvoiceId == purchaseInvoiceId && pip.TenantId == tenantId);

                var paymentHistory = await paymentHistoryQuery.ToListAsync(ct);

                // Step 3: Map to view models for safe response shaping.
                var responseVm = _mapper.Map<List<ReceivedInvoicePaymentVM>>(paymentHistory);

                _logger.LogInfo("Purchase Invoice payment history retrieved successfully. InvoiceId: {InvoiceId}, TenantId: {TenantId}, EmployeeId: {EmployeeId}, PaymentsCount: {PaymentsCount}",
                    purchaseInvoiceId, tenantId, loggedInEmployee.Id, responseVm.Count);

                // Even if there is no payment history, return 200 with empty collection.
                return ApiResponse<object>.SuccessResponse(responseVm, "Purchase invoice payment history retrieved successfully.", 200);
            }
            catch (OperationCanceledException)
            {
                // Explicitly handle cancellation to avoid logging it as an error.
                _logger.LogWarning("GetPurchaseInvoiceHistoryListAsync operation was canceled. InvoiceId: {InvoiceId}, TenantId: {TenantId}, EmployeeId: {EmployeeId}",
                    purchaseInvoiceId, tenantId, loggedInEmployee.Id);

                return ApiResponse<object>.ErrorResponse("The operation was canceled.", "GetPurchaseInvoiceHistoryListAsync was canceled by the caller.", 499);
            }
            catch (Exception ex)
            {
                // Catch-all to ensure no unhandled exception reaches the client.
                _logger.LogError(ex, "Unexpected error while retrieving Purchase Invoice payment history. InvoiceId: {InvoiceId}, TenantId: {TenantId}, EmployeeId: {EmployeeId}",
                    purchaseInvoiceId, tenantId, loggedInEmployee.Id);

                return ApiResponse<object>.ErrorResponse("An unexpected error occurred while retrieving purchase invoice history.", "Unhandled exception in GetPurchaseInvoiceHistoryListAsync.", 500);
            }
        }

        /// <summary>
        /// Adds a payment entry against an existing Purchase Invoice with full validation
        /// and structured logging suitable for enterprise scenarios.
        /// </summary>
        /// <param name="model">Payment details to be recorded against the invoice.</param>
        /// <param name="loggedInEmployee">The currently logged-in employee performing this action.</param>
        /// <param name="tenantId">Current tenant identifier to enforce multi-tenancy boundaries.</param>
        /// <param name="ct">Cancellation token for cooperative cancellation.</param>
        /// <returns>Standardized ApiResponse with the created payment view model or error details.</returns>
        public async Task<ApiResponse<object>> AddPurchaseInvoicePaymentAsync(ReceivedInvoicePaymentDto model, Employee loggedInEmployee, Guid tenantId, CancellationToken ct)
        {
            // Guard clauses to fail fast on invalid input and avoid null reference issues.
            if (model == null)
            {
                _logger.LogWarning("AddPurchaseInvoicePaymentAsync called with null model. TenantId: {TenantId}, EmployeeId: {EmployeeId}", tenantId, loggedInEmployee.Id);
                return ApiResponse<object>.ErrorResponse("Invalid payment data.", "Received null payment model.", 400);
            }

            if (model.InvoiceId == Guid.Empty)
            {
                _logger.LogWarning("AddPurchaseInvoicePaymentAsync called with empty InvoiceId. TenantId: {TenantId}, EmployeeId: {EmployeeId}", tenantId, loggedInEmployee.Id);
                return ApiResponse<object>.ErrorResponse("Invoice reference is required.", "InvoiceId is empty in payment model.", 200);
            }

            if (model.Amount <= 0)
            {
                _logger.LogWarning("AddPurchaseInvoicePaymentAsync called with non-positive Amount. TenantId: {TenantId}, EmployeeId: {EmployeeId}, Amount: {Amount}", tenantId, loggedInEmployee.Id, model.Amount);
                return ApiResponse<object>.ErrorResponse("Payment amount must be greater than zero.", $"Invalid payment amount: {model.Amount}.", 400);
            }

            try
            {
                // Check permissions
                var manageInvoicesPermission = await HasPermissionAsync(PermissionsMaster.ManagePurchaseInvoice, loggedInEmployee.Id);
                if (!manageInvoicesPermission)
                {
                    _logger.LogWarning("Access Denied: {EmployeeId} do not have permission to add payments to a purchase invoice", loggedInEmployee.Id);
                    return ApiResponse<object>.ErrorResponse("Access Denied", "You do not have permission to add payments to a purchase invoice", 403);
                }

                // Create a short-lived DbContext instance using the factory to ensure proper scope per operation.
                await using var context = await _dbContextFactory.CreateDbContextAsync(ct);

                // Step 1: Validate that the invoice exists for the current tenant.
                var purchaseInvoice = await context.PurchaseInvoiceDetails
                    .AsNoTracking()
                    .FirstOrDefaultAsync(
                        pi => pi.Id == model.InvoiceId && pi.TenantId == tenantId,
                        ct);

                if (purchaseInvoice == null)
                {
                    _logger.LogWarning("Purchase Invoice not found. InvoiceId: {InvoiceId}, TenantId: {TenantId}", model.InvoiceId, tenantId);

                    return ApiResponse<object>.ErrorResponse("Purchase invoice not found.", $"Purchase invoice not found for InvoiceId: {model.InvoiceId}, TenantId: {tenantId}.", 404);
                }

                // Step 2: Validate Payment Adjustment Head.
                var paymentAdjustmentHead = await context.PaymentAdjustmentHeads
                    .FirstOrDefaultAsync(pah => pah.Id == model.PaymentAdjustmentHeadId && pah.TenantId == tenantId, ct);

                if (paymentAdjustmentHead == null)
                {
                    _logger.LogWarning("Payment Adjustment Head not found. PaymentAdjustmentHeadId: {PaymentAdjustmentHeadId}, TenantId: {TenantId}", model.PaymentAdjustmentHeadId, tenantId);

                    return ApiResponse<object>.ErrorResponse("Payment adjustment head not found.", $"Payment adjustment head not found for Id: {model.PaymentAdjustmentHeadId}, TenantId: {tenantId}.",
                        404);
                }

                // Step 3: Get existing payments and ensure the new payment does not exceed the invoice total.
                var existingPayments = await context.PurchaseInvoicePayments
                    .Where(pi => pi.InvoiceId == model.InvoiceId && pi.TenantId == tenantId)
                    .ToListAsync(ct);

                var alreadyPaidAmount = existingPayments.Sum(pi => pi.Amount);
                var proposedTotalPaidAmount = alreadyPaidAmount + model.Amount;
                bool proformaCheck = purchaseInvoice.ProformaInvoiceAmount.HasValue ? proposedTotalPaidAmount > purchaseInvoice.ProformaInvoiceAmount : true;
                if (proposedTotalPaidAmount > purchaseInvoice.TotalAmount && proformaCheck)
                {
                    _logger.LogWarning("Attempt to add payment exceeding invoice total. InvoiceId: {InvoiceId}, TenantId: {TenantId}, InvoiceTotal: {InvoiceTotal}, AlreadyPaid: {AlreadyPaid}, NewAmount: {NewAmount}, ProposedTotal: {ProposedTotal}",
                        model.InvoiceId, tenantId, purchaseInvoice.TotalAmount, alreadyPaidAmount, model.Amount, proposedTotalPaidAmount);

                    return ApiResponse<object>.ErrorResponse("Total payment amount cannot exceed the invoice amount.", "Payment addition rejected due to exceeding invoice total amount.",
                        400);
                }

                // Step 4: Map DTO to entity and initialize metadata.
                var receivedInvoicePayment = _mapper.Map<PurchaseInvoicePayment>(model);
                receivedInvoicePayment.Id = Guid.NewGuid();
                receivedInvoicePayment.CreatedAt = DateTime.UtcNow;
                receivedInvoicePayment.CreatedById = loggedInEmployee.Id;
                receivedInvoicePayment.TenantId = tenantId;

                // Step 5: Persist the new payment record.
                context.PurchaseInvoicePayments.Add(receivedInvoicePayment);

                // For enterprise robustness, pass the cancellation token to SaveChangesAsync.
                var saveResult = await context.SaveChangesAsync(ct);

                if (saveResult <= 0)
                {
                    _logger.LogError(null, "SaveChangesAsync returned 0 while adding Purchase Invoice payment. InvoiceId: {InvoiceId}, TenantId: {TenantId}, PaymentId: {PaymentId}",
                        model.InvoiceId, tenantId, receivedInvoicePayment.Id);

                    return ApiResponse<object>.ErrorResponse("Failed to add payment due to a persistence issue.", "Database SaveChangesAsync returned 0 rows affected while adding PurchaseInvoicePayment.",
                        statusCode: StatusCodes.Status500InternalServerError);
                }

                // Step 6: Map entity back to a response view model.
                var responseVm = _mapper.Map<ReceivedInvoicePaymentVM>(receivedInvoicePayment);
                responseVm.PaymentAdjustmentHead = _mapper.Map<PaymentAdjustmentHeadVM>(paymentAdjustmentHead);

                _logger.LogInfo("Purchase Invoice payment added successfully. InvoiceId: {InvoiceId}, TenantId: {TenantId}, PaymentId: {PaymentId}, Amount: {Amount}, EmployeeId: {EmployeeId}",
                    model.InvoiceId, tenantId, receivedInvoicePayment.Id, receivedInvoicePayment.Amount, loggedInEmployee.Id);

                return ApiResponse<object>.SuccessResponse(responseVm, "Payment has been recorded successfully.", 201); // 201 Created is more appropriate for new resource.
            }
            catch (OperationCanceledException)
            {
                // Explicitly handle cancellation to avoid logging it as an error.
                _logger.LogError(null, "AddPurchaseInvoicePaymentAsync operation was canceled. InvoiceId: {InvoiceId}, TenantId: {TenantId}, EmployeeId: {EmployeeId}",
                    model.InvoiceId, tenantId, loggedInEmployee.Id);

                return ApiResponse<object>.ErrorResponse("The operation was canceled.", "AddPurchaseInvoicePaymentAsync was canceled by the caller.", 499); // 499 used by some systems for client cancellation.
            }
            catch (DbUpdateException dbEx)
            {
                // Database-related exceptions with structured logging for observability.
                _logger.LogError(dbEx, "Database update error while adding Purchase Invoice payment. InvoiceId: {InvoiceId}, TenantId: {TenantId}, EmployeeId: {EmployeeId}",
                    model.InvoiceId, tenantId, loggedInEmployee.Id);

                return ApiResponse<object>.ErrorResponse("An error occurred while saving the payment.", "Database update exception occurred during payment creation.", 500);
            }
            catch (Exception ex)
            {
                // Catch-all to avoid leaking unhandled exceptions to the client.
                _logger.LogError(ex, "Unexpected error while adding Purchase Invoice payment. InvoiceId: {InvoiceId}, TenantId: {TenantId}, EmployeeId: {EmployeeId}",
                    model.InvoiceId, tenantId, loggedInEmployee.Id);

                return ApiResponse<object>.ErrorResponse("An unexpected error occurred while processing the payment.", "Unhandled exception in AddPurchaseInvoicePaymentAsync.", 500);
            }
        }

        #endregion

        #region =================================================================== Helper Functions ===================================================================

        /// <summary>
        /// Async permission check helper with scoped DI lifetime
        /// </summary>
        private async Task<bool> HasPermissionAsync(Guid permission, Guid employeeId)
        {
            using var scope = _serviceScopeFactory.CreateScope();
            var permissionService = scope.ServiceProvider.GetRequiredService<PermissionServices>();
            return await permissionService.HasPermission(permission, employeeId);
        }

        public async Task<PurchaseInvoiceDetails?> GetPurchaseInvoiceByIdAsync(Guid id, Guid tenantId, CancellationToken ct = default)
        {
            await using var readContext = await _dbContextFactory.CreateDbContextAsync(ct);
            var purchaseInvoice = await readContext.PurchaseInvoiceDetails
                .Where(e => e.Id == id && e.TenantId == tenantId && e.IsActive)
                .FirstOrDefaultAsync(ct);

            if (purchaseInvoice == null)
            {
                _logger.LogWarning("Purchase Invoice not found. ID: {Id}, TenantID: {TenantId}", id, tenantId);
            }
            else
            {
                _logger.LogInfo("Purchase Invoice found. ID: {Id}, TenantID: {TenantId}", id, tenantId);
            }

            return purchaseInvoice;
        }
        private AdvanceFilter? TryDeserializeFilter(string? filter)
        {
            if (string.IsNullOrWhiteSpace(filter))
            {
                return null;
            }

            var options = new JsonSerializerOptions { PropertyNameCaseInsensitive = true };
            AdvanceFilter? advanceFilter = null;

            try
            {
                // First, try to deserialize directly. This is the expected case (e.g., from a web client).
                advanceFilter = JsonSerializer.Deserialize<AdvanceFilter>(filter, options);
            }
            catch (JsonException ex)
            {
                _logger.LogError(ex, "[{MethodName}] Failed to directly deserialize filter. Attempting to unescape and re-parse. Filter: {Filter}", nameof(TryDeserializeFilter), filter);

                // If direct deserialization fails, it might be an escaped string (common with tools like Postman or some mobile clients).
                try
                {
                    // Unescape the string first, then deserialize the result.
                    string unescapedJsonString = JsonSerializer.Deserialize<string>(filter, options) ?? "";
                    if (!string.IsNullOrWhiteSpace(unescapedJsonString))
                    {
                        advanceFilter = JsonSerializer.Deserialize<AdvanceFilter>(unescapedJsonString, options);
                    }
                }
                catch (JsonException ex1)
                {
                    // If both attempts fail, log the final error and return null.
                    _logger.LogError(ex1, "[{MethodName}] All attempts to deserialize the filter failed. Filter will be ignored. Filter: {Filter}", nameof(TryDeserializeFilter), filter);
                    return null;
                }
            }
            return advanceFilter;
        }

        /// <summary>
        /// Helper method to load infrastructure project by id.
        /// </summary>
        private async Task<BasicProjectVM?> LoadInfraProjectAsync(Guid projectId, Guid tenantId)
        {
            await using var context = await _dbContextFactory.CreateDbContextAsync();
            return await context.Projects.Where(p => p.Id == projectId && p.TenantId == tenantId).Select(p => _mapper.Map<BasicProjectVM>(p)).FirstOrDefaultAsync();
        }

        /// <summary>
        /// Helper method to load service project by id.
        /// </summary>
        private async Task<BasicProjectVM?> LoadServiceProjectAsync(Guid projectId, Guid tenantId)
        {
            await using var context = await _dbContextFactory.CreateDbContextAsync();
            return await context.ServiceProjects.AsNoTracking().Where(sp => sp.Id == projectId && sp.TenantId == tenantId).Select(sp => _mapper.Map<BasicProjectVM>(sp)).FirstOrDefaultAsync();
        }

        private async Task DeleteAttachemnts(List<Guid> documentIds, CancellationToken ct)
        {
            var attachmentTask = Task.Run(async () =>
            {
                await using var context = await _dbContextFactory.CreateDbContextAsync();
                var attachments = await context.PurchaseInvoiceAttachments.AsNoTracking().Where(ba => documentIds.Contains(ba.DocumentId)).ToListAsync(ct);

                context.PurchaseInvoiceAttachments.RemoveRange(attachments);
                await context.SaveChangesAsync(ct);
            });
            var documentsTask = Task.Run(async () =>
            {
                using var scope = _serviceScopeFactory.CreateScope();
                var _updateLogHelper = scope.ServiceProvider.GetRequiredService<UtilityMongoDBHelper>();

                await using var context = await _dbContextFactory.CreateDbContextAsync();
                var documents = await context.Documents.AsNoTracking().Where(ba => documentIds.Contains(ba.Id)).ToListAsync(ct);

                if (documents.Any())
                {
                    context.Documents.RemoveRange(documents);
                    await context.SaveChangesAsync(ct);

                    List<S3DeletionObject> deletionObject = new List<S3DeletionObject>();
                    foreach (var document in documents)
                    {
                        deletionObject.Add(new S3DeletionObject
                        {
                            Key = document.S3Key
                        });
                        if (!string.IsNullOrWhiteSpace(document.ThumbS3Key) && document.ThumbS3Key != document.S3Key)
                        {
                            deletionObject.Add(new S3DeletionObject
                            {
                                Key = document.ThumbS3Key
                            });
                        }
                    }
                    await _updateLogHelper.PushToS3DeletionAsync(deletionObject);
                }
            });

            await Task.WhenAll(attachmentTask, documentsTask);
        }
        #endregion

    }
}