django can we select related a field on a prefetch related model?

Question

Assuming these as django model for the sake of simplcity:

class A():      a = manytomany('B')  class B():      b = charfield()     z = foreignkey('C')  class C():      c = charfield() 

Can we do something like this to fetch the z also:

foo = A.objects.get(pk = 1).prefetch_related('a').select_related('a__z') 

Answer 1

This answer is correct with versions of Django prior to 1.7. It produces three queries: first, fetch the instance of A, then fetch its related instances of B and finally fetch the instances of C related to those of B fetched in the second query.

Before Django 1.7, this is the best you can do, even though the second query could, in theory, select all B objects together with the related C objects joined through the z ForeignKey.

Starting with Django 1.7, there's a more advanced django.db.models.Prefetch class which allows you to do just that. With Prefetch you can customize the queryset used to prefetch related objects like this:

foo = A.objects.prefetch_related(     Prefetch('a', queryset=B.objects.select_related('z')) ).get(pk=1) 

This results in only two queries (as opposed to three when using prefetch_related('a__z')) and lets the database take care of the second join, which should in theory result in slightly better performance.

Answer 2

You only need one prefetch_related call:

foo = A.objects.prefetch_related('a__z').get(pk=1) 

This will prefetch both tables. In Django 1.7+ you can improve performance by using a Prefetch object, as in koniiiik's answer.