ersatztv/ErsatzTV.Application/MediaCollections/Queries/GetCollectionItemsHandler.cs

using ErsatzTV.Application.LibraryBrowse;
using ErsatzTV.Core;
using ErsatzTV.Core.Api.LibraryBrowse;
using ErsatzTV.Core.Errors;
using ErsatzTV.Infrastructure.Data;
using Microsoft.EntityFrameworkCore;

namespace ErsatzTV.Application.MediaCollections;

public class GetCollectionItemsHandler(IDbContextFactory<TvContext> dbContextFactory)
    : IRequestHandler<GetCollectionItems, Either<BaseError, PagedLibraryBrowseItemsResponseModel>>
{
    public async Task<Either<BaseError, PagedLibraryBrowseItemsResponseModel>> Handle(
        GetCollectionItems request,
        CancellationToken cancellationToken)
    {
        await using TvContext dbContext = await dbContextFactory.CreateDbContextAsync(cancellationToken);

        // A null flag here means the collection row does not exist (the projection yields no row),
        // which lets a single query serve both the existence check and the custom-order flag.
        bool? useCustomPlaybackOrder = await dbContext.Collections
            .AsNoTracking()
            .Where(c => c.Id == request.Id)
            .Select(c => (bool?)c.UseCustomPlaybackOrder)
            .SingleOrDefaultAsync(cancellationToken);
        if (useCustomPlaybackOrder is null)
        {
            return new NotFoundError($"Collection {request.Id} does not exist.");
        }

        // The collection graph is bounded, so load every member (with its CustomIndex) and hydrate
        // them in one shared pass (LibraryBrowseItemMapper), then order + page in-memory. Mixed media
        // kinds are supported because MediaItem ids are globally unique across kinds.
        var collectionItems = await dbContext.CollectionItems
            .AsNoTracking()
            .Where(ci => ci.CollectionId == request.Id)
            .Select(ci => new { ci.MediaItemId, ci.CustomIndex })
            .ToListAsync(cancellationToken);

        List<int> mediaItemIds = collectionItems.Select(ci => ci.MediaItemId).ToList();

        List<LibraryBrowseItemResponseModel> all =
            await LibraryBrowseItemMapper.HydrateMediaItemsByIds(dbContext, mediaItemIds, cancellationToken);

        List<LibraryBrowseItemResponseModel> ordered;
        if (useCustomPlaybackOrder.Value)
        {
            // Custom order: sort by CustomIndex (items without one sort last), then title/id as a
            // stable tiebreak.
            var customIndexByMediaItemId = collectionItems
                .GroupBy(ci => ci.MediaItemId)
                .ToDictionary(g => g.Key, g => g.Select(ci => ci.CustomIndex).FirstOrDefault());

            ordered = all
                .OrderBy(i => customIndexByMediaItemId.TryGetValue(i.Id, out int? customIndex) && customIndex.HasValue
                    ? customIndex.Value
                    : int.MaxValue)
                .ThenBy(i => i.Title, StringComparer.OrdinalIgnoreCase)
                .ThenBy(i => i.Id)
                .ToList();
        }
        else
        {
            // Stable title ordering mirrors the library-browse handler (which orders its rows by name),
            // giving the SPA a deterministic, browsable list independent of collection insertion order.
            ordered = all
                .OrderBy(i => i.Title, StringComparer.OrdinalIgnoreCase)
                .ThenBy(i => i.Id)
                .ToList();
        }

        int pageNum = Math.Max(0, request.PageNum);
        int pageSize = Math.Clamp(request.PageSize, 1, 100);

        List<LibraryBrowseItemResponseModel> page = ordered
            .Skip(pageNum * pageSize)
            .Take(pageSize)
            .ToList();

        return new PagedLibraryBrowseItemsResponseModel(ordered.Count, page);
    }
}