[1.8.x] Fixed #23940 -- Allowed model fields to be named `exact`.

An explicit `__exact` lookup in the related managers filters
was interpreted as a reference to a foreign `exact` field.

Thanks to Trac alias zhiyajun11 for the report, Josh for the investigation,
Loïc for the test name and Tim for the review.

Backport of eb4cdfbdd6 from master

django/db/models/fields/related.py

@@ -681,7 +681,7 @@ def create_foreign_related_manager(superclass, rel_field, rel_model):
         def __init__(self, instance):
             super(RelatedManager, self).__init__()
             self.instance = instance
-            self.core_filters = {'%s__exact' % rel_field.name: instance}
+            self.core_filters = {rel_field.name: instance}
             self.model = rel_model
 
         def __call__(self, **kwargs):

tests/managers_regress/models.py

@@ -129,6 +129,7 @@ class Child7(Parent):
 @python_2_unicode_compatible
 class RelatedModel(models.Model):
     test_gfk = GenericRelation('RelationModel', content_type_field='gfk_ctype', object_id_field='gfk_id')
+    exact = models.NullBooleanField()
 
     def __str__(self):
         return force_text(self.pk)
@@ -140,8 +141,8 @@ class RelationModel(models.Model):
 
     m2m = models.ManyToManyField(RelatedModel, related_name='test_m2m')
 
-    gfk_ctype = models.ForeignKey(ContentType)
-    gfk_id = models.IntegerField()
+    gfk_ctype = models.ForeignKey(ContentType, null=True)
+    gfk_id = models.IntegerField(null=True)
     gfk = GenericForeignKey(ct_field='gfk_ctype', fk_field='gfk_id')
 
     def __str__(self):

tests/managers_regress/tests.py

@@ -201,3 +201,11 @@ class ManagersRegressionTests(TestCase):
             t.render(Context({'related': related})),
             ''.join([force_text(relation.pk)] * 3),
         )
+
+    def test_field_can_be_called_exact(self):
+        # Make sure related managers core filters don't include an
+        # explicit `__exact` lookup that could be interpreted as a
+        # reference to a foreign `exact` field. refs #23940.
+        related = RelatedModel.objects.create(exact=False)
+        relation = related.test_fk.create()
+        self.assertEqual(related.test_fk.get(), relation)