Fixed #13085 -- Don't fail on creation of model with GFK to a model with __len__() returning zero.

Also, according to the comments on the ticket and its duplicates, added
tests execising saving an instance of a model with a GFK to:

* An unsaved object -- This actually doesn't generate the same failure
  but another ORM-level exception. The test verifies it's the case.

* An instance of a model with a __nonzero__() method thant returns False
  for it. This doesn't fail because that code path isn't executed.

* An instance of a model with a CharField PK and an empty value for it.
  This doesn't fail.

django/contrib/contenttypes/generic.py

@@ -43,7 +43,7 @@ class GenericForeignKey(object):
 
     def instance_pre_init(self, signal, sender, args, kwargs, **_kwargs):
         """
-        Handles initializing an object with the generic FK instaed of
+        Handles initializing an object with the generic FK instead of
         content-type/object-id fields.
         """
         if self.name in kwargs:
@@ -52,7 +52,7 @@ class GenericForeignKey(object):
             kwargs[self.fk_field] = value._get_pk_val()
 
     def get_content_type(self, obj=None, id=None, using=None):
-        if obj:
+        if obj is not None:
             return ContentType.objects.db_manager(obj._state.db).get_for_model(obj)
         elif id:
             return ContentType.objects.db_manager(using).get_for_id(id)

tests/regressiontests/generic_relations_regress/models.py

@@ -91,3 +91,34 @@ class Company(models.Model):
 
     def __str__(self):
         return "Company: %s" % self.name
+
+# For testing #13085 fix, we also use Note model defined above
+class Developer(models.Model):
+    name = models.CharField(max_length=15)
+
+@python_2_unicode_compatible
+class Team(models.Model):
+    name = models.CharField(max_length=15)
+    members = models.ManyToManyField(Developer)
+
+    def __str__(self):
+        return "%s team" % self.name
+
+    def __len__(self):
+        return self.members.count()
+
+class Guild(models.Model):
+    name = models.CharField(max_length=15)
+    members = models.ManyToManyField(Developer)
+
+    def __nonzero__(self):
+        return self.members.count()
+
+class Tag(models.Model):
+    content_type = models.ForeignKey(ContentType, related_name='g_r_r_tags')
+    object_id = models.CharField(max_length=15)
+    content_object = generic.GenericForeignKey()
+    label = models.CharField(max_length=15)
+
+class Board(models.Model):
+    name = models.CharField(primary_key=True, max_length=15)

tests/regressiontests/generic_relations_regress/tests.py

@@ -1,8 +1,10 @@
 from django.db.models import Q
+from django.db.utils import IntegrityError
 from django.test import TestCase
 
 from .models import (Address, Place, Restaurant, Link, CharLink, TextLink,
-    Person, Contact, Note, Organization, OddRelation1, OddRelation2, Company)
+    Person, Contact, Note, Organization, OddRelation1, OddRelation2, Company,
+    Developer, Team, Guild, Tag, Board)
 
 
 class GenericRelationTests(TestCase):
@@ -98,3 +100,39 @@ class GenericRelationTests(TestCase):
         self.assertEqual(len(places), 2)
         self.assertEqual(count_places(p1), 1)
         self.assertEqual(count_places(p2), 1)
+
+    def test_target_model_is_unsaved(self):
+        """Test related to #13085"""
+        # Fails with another, ORM-level error
+        dev1 = Developer(name='Joe')
+        note = Note(note='Deserves promotion', content_object=dev1)
+        self.assertRaisesMessage(IntegrityError,
+                "generic_relations_regress_note.object_id may not be NULL",
+                note.save)
+
+    def test_target_model_len_zero(self):
+        """Test for #13085 -- __len__() returns 0"""
+        team1 = Team.objects.create(name='Backend devs')
+        try:
+            note = Note(note='Deserve a bonus', content_object=team1)
+        except Exception as e:
+            if issubclass(type(e), Exception) and str(e) == 'Impossible arguments to GFK.get_content_type!':
+                self.fail("Saving model with GenericForeignKey to model instance whose __len__ method returns 0 shouldn't fail.")
+            raise e
+        note.save()
+
+    def test_target_model_nonzero_false(self):
+        """Test related to #13085"""
+        # __nonzero__() returns False -- This actually doesn't currently fail.
+        # This test validates that
+        g1 = Guild.objects.create(name='First guild')
+        note = Note(note='Note for guild', content_object=g1)
+        note.save()
+
+    def test_gfk_to_model_with_empty_pk(self):
+        """Test related to #13085"""
+        # Saving model with GenericForeignKey to model instance with an
+        # empty CharField PK
+        b1 = Board.objects.create(name='')
+        tag = Tag(label='VP', content_object=b1)
+        tag.save()