Deferred model leak

[Pantelis Petridis]

Consider the following models

from django.db import models

class ModelA(models.Model):
        a = models.CharField(max_length=15)

class ModelB(models.Model):
        fk = models.ForeignKey(ModelA)

and the following test

from django.test import TestCase
from .models import ModelA

class DeferProblemTestCase(TestCase):
        def test_defer(self):
                a = ModelA.objects.create(a='test')
                with self.assertNumQueries(2):
                        a.delete()

                unrelated_qs = list(ModelA.objects.defer('a'))

                # let's try once again
                a = ModelA.objects.create(a='test')
                with self.assertNumQueries(2):
                        a.delete()

The above test fails under django 1.8.6, as the second call to .delete() will produce 3 queries instead of 2. It seems django somehow caches the deferred model and tries to delete any related ModelB records twice. Except from the overhead, this can lead to random test fails based on the execution order.