Fixed #25129 -- Made model instance defaults work with migrations (refs #24919).

docs/ref/models/fields.txt

@@ -220,6 +220,10 @@ Note that ``lambda``\s cannot be used for field options like ``default``
 because they cannot be :ref:`serialized by migrations <migration-serializing>`.
 See that documentation for other caveats.
 
+For fields like :class:`ForeignKey` that map to model instances, defaults
+should be the value of the field they reference (``pk`` unless
+:attr:`~ForeignKey.to_field` is set) instead of model instances.
+
 The default value is used when new model instances are created and a value
 isn't provided for the field. When the field is a primary key, the default is
 also used when the field is set to ``None``.

tests/delete/models.py

@@ -13,7 +13,7 @@ class R(models.Model):
 
 
 def get_default_r():
-    return R.objects.get_or_create(is_default=True)[0]
+    return R.objects.get_or_create(is_default=True)[0].pk
 
 
 class S(models.Model):

tests/delete/tests.py

@@ -31,7 +31,7 @@ class OnDeleteTests(TestCase):
         a = create_a('setvalue')
         a.setvalue.delete()
         a = A.objects.get(pk=a.pk)
-        self.assertEqual(self.DEFAULT, a.setvalue)
+        self.assertEqual(self.DEFAULT, a.setvalue.pk)
 
     def test_setnull(self):
         a = create_a('setnull')
@@ -43,7 +43,7 @@ class OnDeleteTests(TestCase):
         a = create_a('setdefault')
         a.setdefault.delete()
         a = A.objects.get(pk=a.pk)
-        self.assertEqual(self.DEFAULT, a.setdefault)
+        self.assertEqual(self.DEFAULT, a.setdefault.pk)
 
     def test_setdefault_none(self):
         a = create_a('setdefault_none')

tests/forms_tests/models.py

@@ -72,7 +72,7 @@ class ChoiceOptionModel(models.Model):
 
 
 def choice_default():
-    return ChoiceOptionModel.objects.get_or_create(name='default')[0]
+    return ChoiceOptionModel.objects.get_or_create(name='default')[0].pk
 
 
 def choice_default_list():

tests/model_fields/models.py

@@ -26,7 +26,7 @@ class Foo(models.Model):
 
 
 def get_foo():
-    return Foo.objects.get(id=1)
+    return Foo.objects.get(id=1).pk
 
 
 class Bar(models.Model):