Fixed #26983 -- Fixed isnull filtering on ForeignKey with to_field

Thanks weidwonder for the report.

django/db/models/fields/related_lookups.py

@@ -93,7 +93,7 @@ class RelatedLookupMixin(object):
             # ForeignKey to IntegerField given value 'abc'. The ForeignKey itself
             # doesn't have validation for non-integers, so we must run validation
             # using the target field.
-            if hasattr(self.lhs.output_field, 'get_path_info'):
+            if self.prepare_rhs and hasattr(self.lhs.output_field, 'get_path_info'):
                 # Get the target field. We can safely assume there is only one
                 # as we don't get to the direct value branch otherwise.
                 target_field = self.lhs.output_field.get_path_info()[-1].target_fields[-1]

docs/releases/1.10.1.txt

@@ -26,3 +26,6 @@ Bugfixes
 
 * Fixed a crash if ``request.META['CONTENT_LENGTH']`` is an empty string
   (:ticket:`27005`).
+
+* Fixed the ``isnull`` lookup on a ``ForeignKey`` with its ``to_field``
+  pointing to a ``CharField`` (:ticket:`26983`).

tests/queries/tests.py

@@ -2483,6 +2483,19 @@ class ToFieldTests(TestCase):
             [node1]
         )
 
+    def test_isnull_query(self):
+        apple = Food.objects.create(name="apple")
+        Eaten.objects.create(food=apple, meal="lunch")
+        Eaten.objects.create(meal="lunch")
+        self.assertQuerysetEqual(
+            Eaten.objects.filter(food__isnull=False),
+            ['<Eaten: apple at lunch>']
+        )
+        self.assertQuerysetEqual(
+            Eaten.objects.filter(food__isnull=True),
+            ['<Eaten: None at lunch>']
+        )
+
 
 class ConditionalTests(BaseQuerysetTest):
     """Tests whose execution depend on different environment conditions like