[1.6.x] Fixed #11811 -- Data-loss bug in queryset.update.

It's now forbidden to call queryset.update(field=instance) when instance
hasn't been saved to the database ie. instance.pk is None.

Conflicts:
	tests/queries/tests.py

Backport of b4cd8169 from master.

django/db/models/base.py

@@ -736,6 +736,8 @@ class Model(six.with_metaclass(ModelBase)):
         return getattr(self, cachename)
 
     def prepare_database_save(self, unused):
+        if self.pk is None:
+            raise ValueError("Unsaved model instance %r cannot be used in an ORM query." % self)
         return self.pk
 
     def clean(self):

tests/queries/models.py

@@ -17,9 +17,13 @@ class ProxyCategory(DumbCategory):
     class Meta:
         proxy = True
 
+@python_2_unicode_compatible
 class NamedCategory(DumbCategory):
     name = models.CharField(max_length=10)
 
+    def __str__(self):
+        return self.name
+
 @python_2_unicode_compatible
 class Tag(models.Model):
     name = models.CharField(max_length=10)

tests/queries/tests.py

@@ -13,6 +13,7 @@ from django.db.models.sql.where import WhereNode, EverythingNode, NothingNode
 from django.db.models.sql.datastructures import EmptyResultSet
 from django.test import TestCase, skipUnlessDBFeature
 from django.test.utils import str_prefix
+from django.utils import six
 from django.utils import unittest
 from django.utils.datastructures import SortedDict
 
@@ -957,7 +958,7 @@ class Queries1Tests(BaseQuerysetTest):
         q = NamedCategory.objects.filter(tag__parent__isnull=True)
         self.assertTrue(str(q.query).count('INNER JOIN') == 1)
         self.assertTrue(str(q.query).count('LEFT OUTER JOIN') == 1)
-        self.assertQuerysetEqual( q, ['<NamedCategory: NamedCategory object>'])
+        self.assertQuerysetEqual(q, ['<NamedCategory: Generic>'])
 
     def test_ticket_10790_4(self):
         # Querying across m2m field should not strip the m2m table from join.
@@ -1267,6 +1268,13 @@ class Queries4Tests(BaseQuerysetTest):
         Item.objects.create(name='i1', created=datetime.datetime.now(), note=n1, creator=self.a1)
         Item.objects.create(name='i2', created=datetime.datetime.now(), note=n1, creator=self.a3)
 
+    def test_ticket11811(self):
+        unsaved_category = NamedCategory(name="Other")
+        with six.assertRaisesRegex(self, ValueError,
+                'Unsaved model instance <NamedCategory: Other> '
+                'cannot be used in an ORM query.'):
+            Tag.objects.filter(pk=self.t1.pk).update(category=unsaved_category)
+
     def test_ticket14876(self):
         # Note: when combining the query we need to have information available
         # about the join type of the trimmed "creator__isnull" join. If we