Fix ordering check for related model pk in Meta.ordering

Allow 'pk' as an alias in Meta.ordering when traversing related fields to resolve regression from 440505cb2.

Adds tests for ordering by related model primary keys.

django/db/models/base.py

@@ -1708,6 +1708,10 @@ class Model(metaclass=ModelBase):
             fld = None
             for part in field.split(LOOKUP_SEP):
                 try:
+                    # pk is an alias that won't be found by opts.get_field.
+                    if part == 'pk':
+                        fld = _cls._meta.pk
+                    else:
                         fld = _cls._meta.get_field(part)
                     if fld.is_relation:
                         _cls = fld.get_path_info()[-1].to_opts.model

tests/invalid_models_tests/test_models.py

@@ -836,6 +836,33 @@ class OtherModelTests(SimpleTestCase):
 
         self.assertFalse(Child.check())
 
+    def test_ordering_pointing_to_related_model_pk(self):
+        class Parent(models.Model):
+            pass
+
+        class Child(models.Model):
+            parent = models.ForeignKey(Parent, models.CASCADE)
+
+            class Meta:
+                ordering = ('parent__pk',)
+
+        self.assertFalse(Child.check())
+
+    def test_ordering_pointing_to_two_related_model_pk(self):
+        class Parent2(models.Model):
+            pass
+
+        class Parent1(models.Model):
+            parent2 = models.ForeignKey(Parent2, models.CASCADE)
+
+        class Child(models.Model):
+            parent1 = models.ForeignKey(Parent1, models.CASCADE)
+
+            class Meta:
+                ordering = ('parent1__parent2__pk',)
+
+        self.assertFalse(Child.check())
+
     def test_name_beginning_with_underscore(self):
         class _Model(models.Model):
             pass