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

using System.Threading.Channels;
using ErsatzTV.Application.Playouts;
using ErsatzTV.Application.Search;
using ErsatzTV.Core;
using ErsatzTV.Core.Domain;
using ErsatzTV.Core.Errors;
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 RemoveItemsFromCollectionHandler : IRequestHandler<RemoveItemsFromCollection, Either<BaseError, Unit>>
{
    private readonly ChannelWriter<IBackgroundServiceRequest> _channel;
    private readonly IDbContextFactory<TvContext> _dbContextFactory;
    private readonly IMediaCollectionRepository _mediaCollectionRepository;
    private readonly ChannelWriter<ISearchIndexBackgroundServiceRequest> _searchChannel;

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

    public async Task<Either<BaseError, Unit>> Handle(
        RemoveItemsFromCollection request,
        CancellationToken cancellationToken)
    {
        await using TvContext dbContext = await _dbContextFactory.CreateDbContextAsync(cancellationToken);
        Option<Collection> maybeCollection = await CollectionMustExist(dbContext, request, cancellationToken);
        return await maybeCollection.Match(
            Some: collection => ApplyRemoveItemsRequest(dbContext, request, collection, cancellationToken),
            None: () => Task.FromResult<Either<BaseError, Unit>>(
                new NotFoundError($"Collection {request.MediaCollectionId} does not exist.")));
    }

    private async Task<Either<BaseError, Unit>> ApplyRemoveItemsRequest(
        TvContext dbContext,
        RemoveItemsFromCollection request,
        Collection collection,
        CancellationToken cancellationToken)
    {
        List<int> requestedIds = request.MediaItemIds.Distinct().ToList();
        var itemsToRemove = collection.MediaItems
            .Filter(m => requestedIds.Contains(m.Id))
            .ToList();

        if (itemsToRemove.Count != requestedIds.Count)
        {
            return new NotFoundError("Collection item does not exist.");
        }

        // No-op when nothing is actually removed: don't rotate the ETag or fan out rebuilds — #269.
        if (itemsToRemove.Count == 0)
        {
            return Unit.Default;
        }

        itemsToRemove.ForEach(m => collection.MediaItems.Remove(m));

        // Rotate the collection ETag so an open custom-order editor's If-Match invalidates (#269);
        // force-write past a concurrent replace-all Version bump — this remove takes no If-Match, so a
        // benign race must not 500 (#253/#269 §7a).
        collection.Version++;
        await dbContext.SaveChangesForcingVersion(cancellationToken);

        // post-commit side effect runs on CancellationToken.None so a late request cancellation
        // can't abort it after the commit landed (#254)
        await _searchChannel.WriteAsync(
            new ReindexMediaItems(itemsToRemove.Select(mi => mi.Id).ToArray()),
            CancellationToken.None);

        // refresh all playouts that use this collection
        foreach (int playoutId in await _mediaCollectionRepository.PlayoutIdsUsingCollection(collection.Id))
        {
            await _channel.WriteAsync(new BuildPlayout(playoutId, PlayoutBuildMode.Refresh), CancellationToken.None);
        }

        return Unit.Default;
    }

    private static Task<Option<Collection>> CollectionMustExist(
        TvContext dbContext,
        RemoveItemsFromCollection request,
        CancellationToken cancellationToken) =>
        dbContext.Collections
            .Include(c => c.MediaItems)
            .SelectOneAsync(c => c.Id, c => c.Id == request.MediaCollectionId, cancellationToken)
            .Map(identity);
}