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

using ErsatzTV.Core;
using ErsatzTV.Core.Domain;
using ErsatzTV.Infrastructure.Data;
using ErsatzTV.Infrastructure.Extensions;
using Microsoft.EntityFrameworkCore;

namespace ErsatzTV.Application.MediaCollections;

public class ReplacePlaylistItemsHandler(IDbContextFactory<TvContext> dbContextFactory)
    : IRequestHandler<ReplacePlaylistItems, Either<BaseError, List<PlaylistItemViewModel>>>
{
    public async Task<Either<BaseError, List<PlaylistItemViewModel>>> Handle(
        ReplacePlaylistItems request,
        CancellationToken cancellationToken)
    {
        await using TvContext dbContext = await dbContextFactory.CreateDbContextAsync(cancellationToken);
        Validation<BaseError, Playlist> validation = await Validate(dbContext, request, cancellationToken);

        // Introduce the optimistic-concurrency check as a standalone Either AFTER the validation
        // pipeline (never via Apply) so a PreconditionFailedError survives to a 412 and is not
        // flattened to a generic 422 by Join() (issue #253 / api-conventions §7a).
        // LanguageExtensions.ToEither joins the Seq<BaseError> to a single BaseError (the native
        // Validation.ToEither() would keep Seq and is called explicitly here to avoid that shadow).
        Either<BaseError, Playlist> validated = LanguageExtensions.ToEither(validation)
            .Bind(playlist => playlist.CheckVersion(request.ExpectedVersions));

        return await validated.Match(
            Right: playlist => Persist(dbContext, request, playlist, cancellationToken),
            Left: error => Task.FromResult<Either<BaseError, List<PlaylistItemViewModel>>>(error));
    }

    private static async Task<Either<BaseError, List<PlaylistItemViewModel>>> Persist(
        TvContext dbContext,
        ReplacePlaylistItems request,
        Playlist playlist,
        CancellationToken cancellationToken)
    {
        playlist.Name = request.Name;
        //playlist.DateUpdated = DateTime.UtcNow;

        dbContext.RemoveRange(playlist.Items);
        playlist.Items = request.Items.Map(i => BuildItem(playlist, i.Index, i)).ToList();

        // Unconditional bump: EF emits the root UPDATE only when a scalar actually differs, so a
        // same-value/no-op save would otherwise write no root row and neither fire the concurrency
        // token nor rotate other clients' ETags. Bumping guarantees both on every save (issue #253).
        playlist.Version++;

        // Save through the guard so an EF concurrency failure (a racing writer won between our load
        // and save) maps to 412 rather than surfacing as a 500.
        Either<BaseError, Unit> saved = await dbContext.SaveChangesWithConcurrencyGuard(cancellationToken);
        return saved.Map(_ => playlist.Items.Map(Mapper.ProjectToViewModel).ToList());
    }

    private static PlaylistItem BuildItem(Playlist playlist, int index, ReplacePlaylistItem item) =>
        new()
        {
            PlaylistId = playlist.Id,
            Index = index,
            CollectionType = item.CollectionType,
            CollectionId = item.CollectionId,
            MultiCollectionId = item.MultiCollectionId,
            SmartCollectionId = item.SmartCollectionId,
            MediaItemId = item.MediaItemId,
            PlaybackOrder = item.PlaybackOrder,
            Count = item.Count,
            PlayAll = item.PlayAll,
            IncludeInProgramGuide = item.IncludeInProgramGuide
        };

    private static Task<Validation<BaseError, Playlist>> Validate(
        TvContext dbContext,
        ReplacePlaylistItems request,
        CancellationToken cancellationToken) =>
        PlaylistMustExist(dbContext, request.PlaylistId, cancellationToken)
            .BindT(playlist => CollectionTypesMustBeValid(request, playlist));

    private static Task<Validation<BaseError, Playlist>> PlaylistMustExist(
        TvContext dbContext,
        int playlistId,
        CancellationToken cancellationToken) =>
        dbContext.Playlists
            .Include(b => b.Items)
            .SelectOneAsync(b => b.Id, b => b.Id == playlistId, cancellationToken)
            .Map(o => o.ToValidation<BaseError>("[PlaylistId] does not exist."));

    private static Validation<BaseError, Playlist> CollectionTypesMustBeValid(
        ReplacePlaylistItems request,
        Playlist playlist) =>
        request.Items.Map(item => CollectionTypeMustBeValid(item, playlist)).Sequence().Map(_ => playlist);

    private static Validation<BaseError, Playlist> CollectionTypeMustBeValid(
        ReplacePlaylistItem item,
        Playlist playlist)
    {
        switch (item.CollectionType)
        {
            case CollectionType.Collection:
                if (item.CollectionId is null)
                {
                    return BaseError.New("[Collection] is required for collection type 'Collection'");
                }

                break;
            case CollectionType.TelevisionShow:
                if (item.MediaItemId is null)
                {
                    return BaseError.New("[MediaItem] is required for collection type 'TelevisionShow'");
                }

                break;
            case CollectionType.TelevisionSeason:
                if (item.MediaItemId is null)
                {
                    return BaseError.New("[MediaItem] is required for collection type 'TelevisionSeason'");
                }

                break;
            case CollectionType.Artist:
                if (item.MediaItemId is null)
                {
                    return BaseError.New("[MediaItem] is required for collection type 'Artist'");
                }

                break;
            case CollectionType.MultiCollection:
                if (item.MultiCollectionId is null)
                {
                    return BaseError.New("[MultiCollection] is required for collection type 'MultiCollection'");
                }

                break;
            case CollectionType.SmartCollection:
                if (item.SmartCollectionId is null)
                {
                    return BaseError.New("[SmartCollection] is required for collection type 'SmartCollection'");
                }

                break;
            case CollectionType.Movie:
            case CollectionType.Episode:
            case CollectionType.MusicVideo:
            case CollectionType.OtherVideo:
            case CollectionType.Song:
            case CollectionType.Image:
                if (item.MediaItemId is null)
                {
                    return BaseError.New($"[MediaItem] is required for type '{item.CollectionType}'");
                }

                break;
            case CollectionType.FakeCollection:
            default:
                return BaseError.New("[CollectionType] is invalid");
        }

        return playlist;
    }
}