Django quickly get latest in one to many relation

Question

I have a model in Django like this

class Artist(models.model):
    pass

class Track(models.model):
    artist = models.ForeignKey(Artist, on_delete=models.CASCADE)

I only have a couple hundred Artists that I want to display on a page alongside their latest Track. I can fetch all the Artists quickly, but fetching their latest Track, when there are thousands of tracks, is proving to be expensive.

latest_track = artist.track_set.latest('when') # really expensive

The way I add tracks to an artist allows me to easily set an artists latest track (i.e. I don't have to worry about multiple threads). This made me think I should just store a field Artist#latest_track, but I don't know how to model that because it's not really a OneToOneField, and that's the closest thing I could find.

I also considered using cached_property, but I don't have a good way to invalidate the cache at the right time. Also, cached_property appears to only setup during runtime, so if I restart my app server, it will need to redo all the queries.

What do I do here?

Answer 1

I bet you just need prefetch_related on artists:

for artist in Artist.objects.all().prefetch_related('track_set'):
    latest_track = artist.track_set.latest('when') #should be no db hit

If it is still slow you need to inspect the generated sql (django debug toolbar, db logs) and see what's going on, often the slowness is coming not from where you expect.