通过外键分组，而外键又是另一个“步骤”

Question

In Django, I have an app with models like this:

class Artist(models.Model):
    name = models.CharField()

class Song(models.Model):
    artist = models.ForeignKey(Artist)
    title = models.CharField()

class SongPlay(models.Model):
    song = models.ForeignKey(Song)
    time = models.DateTimeField()

I want to be able to do a chart of the most played songs and the most played artists. How can I use Django's ORM to do a GROUP BY and a COUNT like this?

At the moment, I'm doing something like this:

SongPlay.objects.values('song__id', 'song__artist__name', 'song__title')
    .annotate(Count('song')).order_by('-song__count')[:10]

...to get the top 10 songs, and this:

SongPlay.objects.values('song__artist__id', 'song__artist__name')
    .annotate(Count('song__artist')).order_by('-song__artist__count')[:10]

...to get the top 10 artists.

Is there an easier way to achieve this? I don't really like specifying the values , I'd rather get tuples along the lines of (song_object, count) and (artist_object, count) .

Is there something obvious I've missed? Thanks!

Answer 1

You should start building your queries from the objects that you want to get in the end.

Artist.objects.annotate(played=Count('song__songplay')).order_by('-played')[:10]
Song.objects.annotate(played=Count('songplay')).order_by('-played')[:10]

Then queries will be much clearer.