Give example of annotate function on querysets

[raiderrobert]

One of the most powerful reducer for extra queries that I discovered was the annotate function. For example:

# in models.py
from django.db import models


class Artist(models.Model):
    name = models.CharField(max_length=250)


class Album(models.Model):
   artist = models.ForeignKey(Artist)
   title = models.CharField(max_length=250)
   published = models.DateField()
  


# in views.py
from django.views.generic import ListView
from .models import Artist


class ArtistList(ListView):
    """
    Common set of fields
    """
    model = Artist
    def get_queryset(self):
        queryset = super(ArtistList, self).get_queryset()
        return queryset.annotate(most_recent=Max('album__published')).order_by('-most_recent')

This results in a single query. A person who was unaware of this function may be tempted to use a list comprehension to loop through the albums for each artist in display.

[natecox]

Is QuerySet#annotate not covered in Two Scoops? I could have sworn there was an example in there somewhere. If not, it's certainly worth mentioning.