[1.8.x] Fixed #15321 -- Honored ancestors unique checks.

Thanks to Tim for the review.

Backport of 79f27f2b61 from master

django/db/models/base.py

@@ -939,7 +939,7 @@ class Model(six.with_metaclass(ModelBase)):
         unique_checks = []
 
         unique_togethers = [(self.__class__, self._meta.unique_together)]
-        for parent_class in self._meta.parents.keys():
+        for parent_class in self._meta.get_parent_list():
             if parent_class._meta.unique_together:
                 unique_togethers.append((parent_class, parent_class._meta.unique_together))
 
@@ -959,7 +959,7 @@ class Model(six.with_metaclass(ModelBase)):
         # the list of checks.
 
         fields_with_class = [(self.__class__, self._meta.local_fields)]
-        for parent_class in self._meta.parents.keys():
+        for parent_class in self._meta.get_parent_list():
             fields_with_class.append((parent_class, parent_class._meta.local_fields))
 
         for model_class, fields in fields_with_class:

tests/model_inheritance/models.py

@@ -186,3 +186,24 @@ class Base(models.Model):
 
 class SubBase(Base):
     sub_id = models.IntegerField(primary_key=True)
+
+
+class GrandParent(models.Model):
+    first_name = models.CharField(max_length=80)
+    last_name = models.CharField(max_length=80)
+    email = models.EmailField(unique=True)
+
+    class Meta:
+        unique_together = ('first_name', 'last_name')
+
+
+class Parent(GrandParent):
+    pass
+
+
+class Child(Parent):
+    pass
+
+
+class GrandChild(Child):
+    pass

tests/model_inheritance/tests.py

@@ -2,7 +2,7 @@ from __future__ import unicode_literals
 
 from operator import attrgetter
 
-from django.core.exceptions import FieldError
+from django.core.exceptions import FieldError, ValidationError
 from django.core.management import call_command
 from django.db import connection
 from django.test import TestCase, TransactionTestCase
@@ -12,7 +12,7 @@ from django.utils import six
 from .models import (
     Chef, CommonInfo, ItalianRestaurant, ParkingLot, Place, Post,
     Restaurant, Student, Supplier, Worker, MixinModel,
-    Title, Copy, Base, SubBase)
+    Title, Copy, Base, SubBase, GrandParent, GrandChild)
 
 
 class ModelInheritanceTests(TestCase):
@@ -423,3 +423,33 @@ class InheritanceSameModelNameTests(TransactionTestCase):
     def test_related_name_attribute_exists(self):
         # The Post model doesn't have an attribute called 'attached_%(app_label)s_%(class)s_set'.
         self.assertFalse(hasattr(self.title, 'attached_%(app_label)s_%(class)s_set'))
+
+
+class InheritanceUniqueTests(TestCase):
+    @classmethod
+    def setUpTestData(cls):
+        cls.grand_parent = GrandParent.objects.create(
+            email='grand_parent@example.com',
+            first_name='grand',
+            last_name='parent',
+        )
+
+    def test_unique(self):
+        grand_child = GrandChild(
+            email=self.grand_parent.email,
+            first_name='grand',
+            last_name='child',
+        )
+        msg = 'Grand parent with this Email already exists.'
+        with self.assertRaisesMessage(ValidationError, msg):
+            grand_child.validate_unique()
+
+    def test_unique_together(self):
+        grand_child = GrandChild(
+            email='grand_child@example.com',
+            first_name=self.grand_parent.first_name,
+            last_name=self.grand_parent.last_name,
+        )
+        msg = 'Grand parent with this First name and Last name already exists.'
+        with self.assertRaisesMessage(ValidationError, msg):
+            grand_child.validate_unique()