Django prefetch_related与列表的通用关系而不是对象

Question

class Album:

    songs = models.ManyToManyField(Song)

class Song:

    title = models.CharField()
    ratings = GenericRelation(Rating)
    modified_at = models.DateTimeField(default=timezone.now, db_index=True)


class Rating:

    user = models.ForeignKey(User)
    content_type = models.ForeignKey(ContentType)
    object_id = models.PositiveIntegerField()
    object = generic.GenericForeignKey('content_type', 'object_id')
    time_stamp = models.DateTimeField(auto_now=True, db_index=True)

对于用户，我想列出相册中的歌曲。

我想在用户评价歌曲后显示每首歌曲是否已被修改。

在代码中，

if Rating.objects.filter(user=user,
    content_type=ContentType.objects.get_for_model(song), object_id=song.id, 
    time_stamp__lt=song.modified_at).exists():
    # song has been modified after user has rated it

此刻我有以下内容，查询失败并显示错误 Cannot resolve keyword 'content_type' into field. Choices are: title, ratings, modified_at

我认为content_type隐含了content_type错误所指的ratings = GenericRelation(Rating)。

qs = album.songs.all()

album.songs.prefetch_related(
    Prefetch(
        'ratings',
        queryset=qs.filter(
            ratings__user=user,
            ratings__rating_type=RATING_TYPE_VIEW
        ).annotate(time_stamp=Max('ratings__time_stamp')),
        to_attr='ratings_prefetched'
    )
)

我在django 1.8.4