Duplicated queries with nested prefetch

[Eduardo Klein]

This ticket may be related to #26318 , but it's not the same since it does not involve repeated models.

Consider the following 3 test models:

class Test1(models.Model):
    pass

class Test2(models.Model):
    test1 = models.ForeignKey(Test1, related_name="tests2")

class Test3(models.Model):
    test2 = models.ForeignKey(Test2, related_name="tests3")

Through the shell, we create initial entries and run the nested prefetch queries. We verify that the last 2 queries are duplicated:

from test.models import Test1, Test2, test3
from django.db import connection
from django.db.models import Prefetch

t1 = Test1.objects.create()
t2 = Test2.objects.create(test1 = t1)
t3 = Test3.objects.create(test2 = t2)

connection.queries_log.clear()

tests2_prefetch = Prefetch("tests2", Test2.objects.prefetch_related("tests3"))
Test1.objects.prefetch_related(tests2_prefetch)

assert len(connection.queries) == 4
assert connection.queries[-1] == connection.queries[-2]

We had 4 queries instead of 3 expected and the last 2 are duplicated (to prefetch Test3 entries).

[Tim Graham]

I tested the code from the description and this seems to be a duplicate of #25546 which is fixed in master (which will be Django 1.10).