Django模型和SELECT连接两个或多个表？

Question

如何使用Django模型使用两个(或更多)连接表进行选择？

例如:

class Album(models.Model):
    artist = models.ForeignKey(Musician)
    name = models.CharField(max_length=100)
    release_date = models.DateField()
    num_stars = models.IntegerField()

class Song(models.Model):
    album = models.ForeignKey(Album)
    name = models.CharField(max_length=100)
    num_stars = models.IntegerField()

SELECT * from album, song where (album.id = song.album_id) and (album.artist_id = X)

Answer 1

Django查询集希望返回模型实例的列表,这些列表并不完全映射到从连接到单个结果表的多个表返回数据的SQL概念.

要实现您提供的SQL查询的效果作为示例,您可以执行以下操作之一:

1)如果您想通过歌曲迭代您的数据,您可以通过艺术家ID限制您查询的歌曲,如下所示:

songs = Song.objects.filter(album__artist__id=123)
for song in songs:
    print song.name
    print song.album.name
    ... do what ever you need to do with song here ...

如果您担心性能,可以将select_related()添加 到查询集中.

# the depth argument tells django to only follow foreign key relationships
# from Song one level deep
songs = Song.objects.select_related(depth=1).filter(album__artist__id=123)

for song in songs:
    # django has now already fetched the album data, so accessing it
    # from the song instance does not cause an additional database hit
    print song.album.name

2)如果你想按专辑迭代你的数据,你可以这样做:

albums = Album.objects.filter(artist__id = 123)
for album in albums:
    for song in album.song_set.all():
        print song.name
        print album.name
        ... etc ...