Fixed #30798 -- Fixed Meta.ordering validation for pk of related fields.

Regression in 440505cb2c.

django/db/models/base.py

@@ -1708,7 +1708,11 @@ class Model(metaclass=ModelBase):
             fld = None
             for part in field.split(LOOKUP_SEP):
                 try:
-                    fld = _cls._meta.get_field(part)
+                    # 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
                     else:

tests/invalid_models_tests/test_models.py

@@ -844,6 +844,18 @@ class OtherModelTests(SimpleTestCase):
         with register_lookup(models.CharField, Lower):
             self.assertEqual(Model.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.assertEqual(Child.check(), [])
+
     def test_ordering_pointing_to_foreignkey_field(self):
         class Parent(models.Model):
             pass