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);

        bool exists = await dbContext.Collections
            .AsNoTracking()
            .AnyAsync(c => c.Id == request.Id, cancellationToken);
        if (!exists)
        {
            return new NotFoundError($"Collection {request.Id} does not exist.");
        }

        // The collection graph is bounded, so load every member id 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.
        List<int> mediaItemIds = await dbContext.CollectionItems
            .AsNoTracking()
            .Where(ci => ci.CollectionId == request.Id)
            .Select(ci => ci.MediaItemId)
            .ToListAsync(cancellationToken);

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

        // 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.
        List<LibraryBrowseItemResponseModel> 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);
    }
}