Django chaining prefetch_related and select_related

Question

Let's say I have following models

class Foo(models.Model):
    ...

class Prop(models.Model):
    ...

class Bar(models.Model):
    foo: models.ForeignKey(Foo, related_name='bars', ...)
    prop: models.ForeignKey(Prop, ...)

Now I want to make the following query.

foos = Foo.objects.prefetch_related('bars__prop').all()

Does the above query makes 3 Database calls or only 2 (select_related for prop from bar), given that only one prop is associated with bar

If it takes 3 calls then, is there a way to make it 2 calls by using selected_related for bar -> prop

Answer 1

You can use the Prefetch class to specify the queryset that is used in prefetch_related() and this way combine it with select_related():

from django.db.models import Prefetch
bars = Bar.objects.select_related('prop')
foos = Foo.objects.prefetch_related(Prefetch('bars', queryset=bars)).all()

Note that this should be two queries, one for the Foo objects and one for getting the related Bar objects that are being joined in the same query with Prop.