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

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

namespace ErsatzTV.Application.MediaCollections;

public class UpdateMultiCollectionHandler : IRequestHandler<UpdateMultiCollection, Either<BaseError, Unit>>
{
    private readonly ChannelWriter<IBackgroundServiceRequest> _channel;
    private readonly IDbContextFactory<TvContext> _dbContextFactory;
    private readonly IMediaCollectionRepository _mediaCollectionRepository;
    private readonly ISearchTargets _searchTargets;

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

    public async Task<Either<BaseError, Unit>> Handle(
        UpdateMultiCollection request,
        CancellationToken cancellationToken)
    {
        await using TvContext dbContext = await _dbContextFactory.CreateDbContextAsync(cancellationToken);
        Validation<BaseError, MultiCollection> 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, MultiCollection> 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,
        MultiCollection c,
        UpdateMultiCollection request,
        CancellationToken cancellationToken)
    {
        c.Name = request.Name;

        // Bump the version on this first save (issue #253 §7a): it rotates other clients' ETags and,
        // via IsConcurrencyToken, closes the load→save race — a stale writer's WHERE Version=@orig hits
        // 0 rows → PreconditionFailedError (412). Saving the name first also keeps a name-only change
        // from triggering a playout rebuild (the item save below stays gated on real item changes).
        c.Version++;
        Either<BaseError, Unit> nameSave = await dbContext.SaveChangesWithConcurrencyGuard(cancellationToken);
        if (nameSave.IsLeft)
        {
            return nameSave;
        }

        var toAdd = request.Items
            .Filter(i => i.CollectionId.HasValue)
            // ReSharper disable once PossibleInvalidOperationException
            .Filter(i => c.MultiCollectionItems.All(i2 => i2.CollectionId != i.CollectionId.Value))
            .Map(i => new MultiCollectionItem
            {
                // ReSharper disable once PossibleInvalidOperationException
                CollectionId = i.CollectionId.Value,
                MultiCollectionId = c.Id,
                ScheduleAsGroup = i.ScheduleAsGroup,
                PlaybackOrder = i.PlaybackOrder,
                Weight = i.Weight
            })
            .ToList();
        var toRemove = c.MultiCollectionItems
            .Filter(i => request.Items.All(i2 => i2.CollectionId != i.CollectionId))
            .ToList();

        // remove items that are no longer present
        c.MultiCollectionItems.RemoveAll(toRemove.Contains);

        // update existing items
        foreach (MultiCollectionItem item in c.MultiCollectionItems)
        {
            foreach (UpdateMultiCollectionItem incoming in
                     request.Items.Filter(i => i.CollectionId == item.CollectionId))
            {
                item.ScheduleAsGroup = incoming.ScheduleAsGroup;
                item.PlaybackOrder = incoming.PlaybackOrder;
                item.Weight = incoming.Weight;
            }
        }

        // add new items
        c.MultiCollectionItems.AddRange(toAdd);

        var toAddSmart = request.Items
            .Filter(i => i.SmartCollectionId.HasValue)
            // ReSharper disable once PossibleInvalidOperationException
            .Filter(i => c.MultiCollectionSmartItems.All(i2 => i2.SmartCollectionId != i.SmartCollectionId.Value))
            .Map(i => new MultiCollectionSmartItem
            {
                // ReSharper disable once PossibleInvalidOperationException
                SmartCollectionId = i.SmartCollectionId.Value,
                MultiCollectionId = c.Id,
                ScheduleAsGroup = i.ScheduleAsGroup,
                PlaybackOrder = i.PlaybackOrder,
                Weight = i.Weight
            })
            .ToList();
        var toRemoveSmart = c.MultiCollectionSmartItems
            .Filter(i => request.Items.All(i2 => i2.SmartCollectionId != i.SmartCollectionId))
            .ToList();

        // remove items that are no longer present
        c.MultiCollectionSmartItems.RemoveAll(toRemoveSmart.Contains);

        // update existing items
        foreach (MultiCollectionSmartItem item in c.MultiCollectionSmartItems)
        {
            foreach (UpdateMultiCollectionItem incoming in request.Items.Filter(i =>
                         i.SmartCollectionId == item.SmartCollectionId))
            {
                item.ScheduleAsGroup = incoming.ScheduleAsGroup;
                item.PlaybackOrder = incoming.PlaybackOrder;
                item.Weight = incoming.Weight;
            }
        }

        // add new items
        c.MultiCollectionSmartItems.AddRange(toAddSmart);

        // rebuild playouts
        if (await dbContext.SaveChangesAsync(cancellationToken) > 0)
        {
            _searchTargets.SearchTargetsChanged();

            // refresh all playouts that use this collection
            // post-commit side effect runs on CancellationToken.None so a late request cancellation
            // can't abort it after the commit landed (#254)
            foreach (int playoutId in await _mediaCollectionRepository.PlayoutIdsUsingMultiCollection(
                         request.MultiCollectionId))
            {
                await _channel.WriteAsync(new BuildPlayout(playoutId, PlayoutBuildMode.Refresh), CancellationToken.None);
            }
        }

        return Unit.Default;
    }

    private static async Task<Validation<BaseError, MultiCollection>> Validate(
        TvContext dbContext,
        UpdateMultiCollection request,
        CancellationToken cancellationToken) =>
        (await MultiCollectionMustExist(dbContext, request, cancellationToken),
            await ValidateName(dbContext, request),
            ValidateWeights(request))
        .Apply((collectionToUpdate, _, _) => collectionToUpdate);

    // Bounds are shared with the create path so the two cannot drift. This gate matters more here than on
    // create: EF's HasDefaultValue substitutes 1 for a 0 on INSERT (0 reads as "not set"), but an UPDATE
    // writes the 0 through -- and a 0-weight source is filtered out of the rotation, deleting it from the
    // channel with nothing reported. See #70.
    private static Validation<BaseError, Unit> ValidateWeights(UpdateMultiCollection request) =>
        request.Items.All(i => MultiCollectionItemWeight.IsValid(i.Weight))
            ? Unit.Default
            : BaseError.New(MultiCollectionItemWeight.ValidationMessage);

    private static Task<Validation<BaseError, MultiCollection>> MultiCollectionMustExist(
        TvContext dbContext,
        UpdateMultiCollection updateCollection,
        CancellationToken cancellationToken) =>
        dbContext.MultiCollections
            .Include(mc => mc.MultiCollectionItems)
            .Include(mc => mc.MultiCollectionSmartItems)
            .SelectOneAsync(c => c.Id, c => c.Id == updateCollection.MultiCollectionId, cancellationToken)
            .Map(o => o.ToValidation<BaseError>("MultiCollection does not exist."));

    private static async Task<Validation<BaseError, string>> ValidateName(
        TvContext dbContext,
        UpdateMultiCollection updateMultiCollection)
    {
        Validation<BaseError, string> result1 = updateMultiCollection.NotEmpty(c => c.Name)
            .Bind(_ => updateMultiCollection.NotLongerThan(50)(c => c.Name));

        bool duplicateName = await dbContext.MultiCollections
            .AnyAsync(c => c.Id != updateMultiCollection.MultiCollectionId && c.Name == updateMultiCollection.Name);

        Validation<BaseError, Unit> result2 = duplicateName
            ? Fail<BaseError, Unit>("MultiCollection name must be unique")
            : Success<BaseError, Unit>(Unit.Default);

        return (result1, result2).Apply((_, _) => updateMultiCollection.Name);
    }
}