fix(backend): compute track listens count via SQL instead of an unloaded navigation

TrackApplicationResponse.FromEntity counted track.ListeningHistories,
but none of the queries that built a response ever Included it, so
the collection was always empty and listensCount was always 0. Each
query now projects the count as a correlated subquery before mapping
to the response.

backend/Musify.Application/PlayLists/GetPlayListTracks.cs

@@ -4,6 +4,7 @@ using Microsoft.EntityFrameworkCore;
 using Musify.Application.Contracts;
 using Musify.Application.Shared;
 using Musify.Application.Tracks.Responses;
+using X.PagedList;
 using X.PagedList.EF;
 
 namespace Musify.Application.PlayLists
@@ -32,11 +33,15 @@ namespace Musify.Application.PlayLists
 
             var totalCount = await tracksQuery.CountAsync(cancellationToken);
 
-            var pagedTracks = await tracksQuery
+            var pagedEntities = await tracksQuery
                 .OrderBy(plt => plt.Position)
-                .Select(plt => TrackApplicationResponse.FromEntity(plt.Track))
+                .Select(plt => new { plt.Track, ListensCount = plt.Track.ListeningHistories.Count })
                 .ToPagedListAsync(request.PageNumber, request.PageSize, totalCount, cancellationToken);
 
+            var pagedTracks = new StaticPagedList<TrackApplicationResponse>(
+                pagedEntities.Select(x => TrackApplicationResponse.FromEntity(x.Track, x.ListensCount)),
+                pagedEntities.PageNumber, pagedEntities.PageSize, pagedEntities.TotalItemCount);
+
             return PaginatedResponse<TrackApplicationResponse>.FromPagedList(pagedTracks);
         }
     }

backend/Musify.Application/Tracks/CreateTrack.cs

@@ -123,7 +123,7 @@ namespace Musify.Application.Tracks
                 return Error.Failure(description: $"Failed to create track for {request.Title}");
             }
 
-            return TrackApplicationResponse.FromEntity(trackEntity);
+            return TrackApplicationResponse.FromEntity(trackEntity, listensCount: 0);
         }
     }
 }

backend/Musify.Application/Tracks/GetListeningHistory.cs

@@ -17,10 +17,11 @@ namespace Musify.Application.Tracks
             var listeningHistory = database.ListeningHistories
                 .Where(l => l.UserId == query.UserId)
                 .OrderByDescending(l => l.ListenedAt)
-                .Select(t => TrackApplicationResponse.FromEntity(t.Track))
+                .Select(t => new { t.Track, ListensCount = t.Track.ListeningHistories.Count })
+                .AsEnumerable()
+                .DistinctBy(t => t.Track.Id)
                 .Take(ListSize)
-                .DistinctBy(t => t.Id)
-                .AsEnumerable();
+                .Select(t => TrackApplicationResponse.FromEntity(t.Track, t.ListensCount));
 
             return ValueTask.FromResult(listeningHistory);
         }

backend/Musify.Application/Tracks/GetTrackById.cs

@@ -14,15 +14,15 @@ namespace Musify.Application.Tracks
     {
         public async ValueTask<ErrorOr<TrackApplicationResponse>> Handle(GetTrackByIdQuery request, CancellationToken cancellationToken)
         {
-            var track = await database.Tracks.AsNoTracking()
+            var entity = await database.Tracks.AsNoTracking()
                 .Where(t => t.Id == request.TrackId)
-                .Select(t => TrackApplicationResponse.FromEntity(t))
+                .Select(t => new { Track = t, ListensCount = t.ListeningHistories.Count })
                 .SingleOrDefaultAsync(cancellationToken);
 
-            if (track is null)
+            if (entity is null)
                 return Error.NotFound();
 
-            return track;
+            return TrackApplicationResponse.FromEntity(entity.Track, entity.ListensCount);
         }
     }
 }

backend/Musify.Application/Tracks/GetTracks.cs

@@ -3,6 +3,7 @@ using Mediator;
 using Microsoft.EntityFrameworkCore;
 using Musify.Application.Shared;
 using Musify.Application.Tracks.Responses;
+using X.PagedList;
 using X.PagedList.EF;
 using Musify.Application.Contracts;
 
@@ -29,11 +30,15 @@ namespace Musify.Application.Tracks
 
             var totalCount = await tracksQuery.CountAsync(cancellationToken);
 
-            var pagedTracks = await tracksQuery
+            var pagedEntities = await tracksQuery
                 .OrderBy(t => t.Id)
-                .Select(t => TrackApplicationResponse.FromEntity(t.Track))
+                .Select(t => new { t.Track, ListensCount = t.Track.ListeningHistories.Count })
                 .ToPagedListAsync(request.PageNumber, request.PageSize, totalCount, cancellationToken);
 
+            var pagedTracks = new StaticPagedList<TrackApplicationResponse>(
+                pagedEntities.Select(x => TrackApplicationResponse.FromEntity(x.Track, x.ListensCount)),
+                pagedEntities.PageNumber, pagedEntities.PageSize, pagedEntities.TotalItemCount);
+
             return PaginatedResponse<TrackApplicationResponse>.FromPagedList(pagedTracks);
         }
     }

backend/Musify.Application/Tracks/GetTracksByUserId.cs

@@ -3,6 +3,7 @@ using Mediator;
 using Microsoft.EntityFrameworkCore;
 using Musify.Application.Shared;
 using Musify.Application.Tracks.Responses;
+using X.PagedList;
 using X.PagedList.EF;
 using Musify.Application.Contracts;
 
@@ -36,11 +37,15 @@ namespace Musify.Application.Tracks
 
             var totalCount = await tracksQuery.CountAsync(cancellationToken);
 
-            var pagedTracks = await tracksQuery
+            var pagedEntities = await tracksQuery
                 .OrderBy(t => t.Track.CreatedAt)
-                .Select(t => TrackApplicationResponse.FromEntity(t.Track))
+                .Select(t => new { t.Track, ListensCount = t.Track.ListeningHistories.Count })
                 .ToPagedListAsync(pageNumber, pageSize, totalCount, cancellationToken);
 
+            var pagedTracks = new StaticPagedList<TrackApplicationResponse>(
+                pagedEntities.Select(x => TrackApplicationResponse.FromEntity(x.Track, x.ListensCount)),
+                pagedEntities.PageNumber, pagedEntities.PageSize, pagedEntities.TotalItemCount);
+
             return PaginatedResponse<TrackApplicationResponse>.FromPagedList(pagedTracks);
         }
     }

backend/Musify.Application/Tracks/Responses/TrackApplicationResponse.cs

@@ -10,17 +10,13 @@ namespace Musify.Application.Tracks.Responses
         DateTime CreatedAt,
         DateTime UpdatedAt)
     {
-        public static TrackApplicationResponse FromEntity(Track track)
-        {
-            var count = track.ListeningHistories.Count(x => x.TrackId == track.Id);
-
-            return new TrackApplicationResponse(
+        public static TrackApplicationResponse FromEntity(Track track, int listensCount) =>
+            new(
                 track.Id,
                 track.Title,
                 track.Duration,
-                count,
+                listensCount,
                 track.CreatedAt,
                 track.UpdatedAt);
-        }
     }
 }