вернул всё обратно

server/proj/audio/models.py

@@ -22,7 +22,7 @@ class Song(models.Model):
         return f"{self.artist} - {self.title}"
 
 class FavoriteSong(models.Model):
-    song = models.ManyToManyField(Song, verbose_name='Трек', related_name='song', null=True, blank=True)
+    song = models.ForeignKey(Song, verbose_name='Трек', on_delete=models.CASCADE, null=True, blank=True)
     user = models.ForeignKey(MyUser, verbose_name='Пользователь', on_delete=models.CASCADE, blank=True, null=True)
     def __str__(self):
         return f"{self.song.title}"

server/proj/audio/serializers.py

@@ -14,6 +14,6 @@ class FavoriteSongSerializer(serializers.ModelSerializer):
     def to_representation(self, instance):
         rep = super().to_representation(instance)
         rep["song"] = SongSerializer(
-            instance.song.all(), many=True).data
+            instance.song, many=True).data
         return rep