ersatztv/ErsatzTV.Application/MediaCollections/Commands/UpdateCollectionCustomOrderHandler.cs

using System.Threading.Channels;
using ErsatzTV.Application.Playouts;
using ErsatzTV.Core;
using ErsatzTV.Core.Domain;
using ErsatzTV.Core.Interfaces.Repositories;
using ErsatzTV.Core.Scheduling;
using ErsatzTV.Infrastructure.Data;
using ErsatzTV.Infrastructure.Extensions;
using Microsoft.EntityFrameworkCore;

namespace ErsatzTV.Application.MediaCollections;

public class UpdateCollectionCustomOrderHandler : IRequestHandler<UpdateCollectionCustomOrder, Either<BaseError, Unit>>
{
    private readonly ChannelWriter<IBackgroundServiceRequest> _channel;
    private readonly IDbContextFactory<TvContext> _dbContextFactory;
    private readonly IMediaCollectionRepository _mediaCollectionRepository;

    public UpdateCollectionCustomOrderHandler(
        IDbContextFactory<TvContext> dbContextFactory,
        IMediaCollectionRepository mediaCollectionRepository,
        ChannelWriter<IBackgroundServiceRequest> channel)
    {
        _dbContextFactory = dbContextFactory;
        _mediaCollectionRepository = mediaCollectionRepository;
        _channel = channel;
    }

    public async Task<Either<BaseError, Unit>> Handle(
        UpdateCollectionCustomOrder request,
        CancellationToken cancellationToken)
    {
        await using TvContext dbContext = await _dbContextFactory.CreateDbContextAsync(cancellationToken);
        Validation<BaseError, Collection> validation = await Validate(dbContext, request, cancellationToken);

        // Optimistic-concurrency check as a standalone Either AFTER validation, never via Apply (which
        // Join()s the error Seq and would flatten PreconditionFailedError to a 422) — issue #253 §7a.
        Either<BaseError, Collection> validated = LanguageExtensions.ToEither(validation)
            .Bind(c => c.CheckVersion(request.ExpectedVersions));

        return await validated.Match(
            Right: c => ApplyUpdateRequest(dbContext, c, request, cancellationToken),
            Left: error => Task.FromResult<Either<BaseError, Unit>>(error));
    }

    private async Task<Either<BaseError, Unit>> ApplyUpdateRequest(
        TvContext dbContext,
        Collection c,
        UpdateCollectionCustomOrder request,
        CancellationToken cancellationToken)
    {
        foreach (MediaItemCustomOrder updateItem in request.MediaItemCustomOrders)
        {
            Option<CollectionItem> maybeCollectionItem = c.CollectionItems
                .FirstOrDefault(ci => ci.MediaItemId == updateItem.MediaItemId);

            foreach (CollectionItem collectionItem in maybeCollectionItem)
            {
                collectionItem.CustomIndex = updateItem.CustomIndex;
            }
        }

        // Unconditional bump (issue #253 §7a / M1) then guarded save (→ 412 on a lost race).
        c.Version++;
        Either<BaseError, Unit> saveResult = await dbContext.SaveChangesWithConcurrencyGuard(cancellationToken);
        if (saveResult.IsLeft)
        {
            return saveResult;
        }

        // Refresh all playouts that use this collection. The old `SaveChangesAsync() > 0` gate is always
        // true once the version bumps unconditionally (M2), so run the refresh on any successful save.
        // Post-commit enqueue on CancellationToken.None so a late cancellation can't drop the rebuild
        // after the commit landed (#254 / §7b).
        foreach (int playoutId in await _mediaCollectionRepository
                     .PlayoutIdsUsingCollection(request.CollectionId))
        {
            await _channel.WriteAsync(
                new BuildPlayout(playoutId, PlayoutBuildMode.Refresh),
                CancellationToken.None);
        }

        return Unit.Default;
    }

    private static Task<Validation<BaseError, Collection>> Validate(
        TvContext dbContext,
        UpdateCollectionCustomOrder request,
        CancellationToken cancellationToken) =>
        CollectionMustExist(dbContext, request, cancellationToken);

    private static Task<Validation<BaseError, Collection>> CollectionMustExist(
        TvContext dbContext,
        UpdateCollectionCustomOrder request,
        CancellationToken cancellationToken) =>
        dbContext.Collections
            .Include(c => c.CollectionItems)
            .SelectOneAsync(c => c.Id, c => c.Id == request.CollectionId, cancellationToken)
            .Map(o => o.ToValidation<BaseError>("Collection does not exist."));
}