Nexus.Reader/src/NexusReader.Application/Features/Books/Commands/PublishBookVersionCommandHandler.cs

using System;
using System.Threading;
using System.Threading.Tasks;
using FluentResults;
using MediatR;
using Microsoft.EntityFrameworkCore;
using NexusReader.Application.Abstractions.Messaging;
using NexusReader.Data.Persistence;
using NexusReader.Domain.Entities;
using NexusReader.Domain.Exceptions;

namespace NexusReader.Application.Features.Books.Commands;

/// <summary>
/// MediatR handler for publishing a Book version and setting up the next Working Draft.
/// </summary>
public class PublishBookVersionCommandHandler : ICommandHandler<PublishBookVersionCommand>
{
    private readonly IDbContextFactory<AppDbContext> _dbContextFactory;

    public PublishBookVersionCommandHandler(IDbContextFactory<AppDbContext> dbContextFactory)
    {
        _dbContextFactory = dbContextFactory;
    }

    public async Task<Result> Handle(PublishBookVersionCommand request, CancellationToken cancellationToken)
    {
        using var dbContext = await _dbContextFactory.CreateDbContextAsync(cancellationToken);

        // Fetch the Book including its CurrentDraftRevision and all associated Chapters,
        // enforcing that the book belongs to the requested TenantId and UserId to prevent cross-tenant data leaks.
        var book = await dbContext.Books
            .Include(b => b.CurrentDraftRevision)
                .ThenInclude(r => r!.Chapters)
            .FirstOrDefaultAsync(
                b => b.Id == request.BookId && b.UserId == request.UserId && b.TenantId == request.TenantId, 
                cancellationToken);

        if (book == null)
        {
            throw new BookNotFoundException(request.BookId);
        }

        var oldDraftRevision = book.CurrentDraftRevision;
        if (oldDraftRevision == null)
        {
            return Result.Fail(new Error("The book does not have an active draft revision to publish."));
        }

        // Start ACID transaction
        using var transaction = await dbContext.Database.BeginTransactionAsync(cancellationToken);
        try
        {
            // 1. Update the current draft revision: Set IsPublished = true, PublishedAt = now, VersionString = custom
            oldDraftRevision.IsPublished = true;
            oldDraftRevision.PublishedAt = DateTime.UtcNow;
            oldDraftRevision.VersionString = request.CustomVersionString;

            // 2. Point the Book.LivePublishedRevisionId to this newly frozen revision ID
            book.LivePublishedRevisionId = oldDraftRevision.Id;

            // 3. Execute Deep Snapshot: Instantiate a brand new BookRevision representing the next "Working Draft"
            var newDraftRevision = new BookRevision
            {
                Id = Guid.NewGuid(),
                BookId = book.Id,
                VersionString = "Working Draft",
                IsPublished = false,
                CreatedAt = DateTime.UtcNow
            };

            dbContext.BookRevisions.Add(newDraftRevision);

            // Replicate/clone chapters into new Chapter objects associated with the new draft revision.
            // Reset identities by explicitly instantiating completely new Chapter objects with Guid.NewGuid().
            foreach (var oldChapter in oldDraftRevision.Chapters)
            {
                var newChapter = new Chapter
                {
                    Id = Guid.NewGuid(),
                    BookRevisionId = newDraftRevision.Id,
                    Title = oldChapter.Title,
                    MarkdownContent = oldChapter.MarkdownContent,
                    SortOrder = oldChapter.SortOrder
                };
                dbContext.Chapters.Add(newChapter);
            }

            // 4. Assign the new draft revision ID to Book.CurrentDraftRevisionId
            book.CurrentDraftRevisionId = newDraftRevision.Id;

            // Save changes and commit transaction
            await dbContext.SaveChangesAsync(cancellationToken);
            await transaction.CommitAsync(cancellationToken);

            return Result.Ok();
        }
        catch (Exception ex)
        {
            try
            {
                await transaction.RollbackAsync(cancellationToken);
            }
            catch (Exception rollbackEx)
            {
                Console.WriteLine($"[PublishBookVersion] Transaction rollback failed: {rollbackEx.Message}");
            }

            return Result.Fail(new Error($"Failed to publish book version: {ex.Message}").CausedBy(ex));
        }
    }
}