Fixed #470 -- Added support for database defaults on fields.

Special thanks to Hannes Ljungberg for finding multiple implementation
gaps.

Thanks also to Simon Charette, Adam Johnson, and Mariusz Felisiak for
reviews.

tests/schema/tests.py

@@ -2102,6 +2102,33 @@ class SchemaTests(TransactionTestCase):
         with self.assertRaises(IntegrityError):
             NoteRename.objects.create(detail_info=None)
 
+    @isolate_apps("schema")
+    def test_rename_keep_db_default(self):
+        """Renaming a field shouldn't affect a database default."""
+
+        class AuthorDbDefault(Model):
+            birth_year = IntegerField(db_default=1985)
+
+            class Meta:
+                app_label = "schema"
+
+        self.isolated_local_models = [AuthorDbDefault]
+        with connection.schema_editor() as editor:
+            editor.create_model(AuthorDbDefault)
+        columns = self.column_classes(AuthorDbDefault)
+        self.assertEqual(columns["birth_year"][1].default, "1985")
+
+        old_field = AuthorDbDefault._meta.get_field("birth_year")
+        new_field = IntegerField(db_default=1985)
+        new_field.set_attributes_from_name("renamed_year")
+        new_field.model = AuthorDbDefault
+        with connection.schema_editor(
+            atomic=connection.features.supports_atomic_references_rename
+        ) as editor:
+            editor.alter_field(AuthorDbDefault, old_field, new_field, strict=True)
+        columns = self.column_classes(AuthorDbDefault)
+        self.assertEqual(columns["renamed_year"][1].default, "1985")
+
     @skipUnlessDBFeature(
         "supports_column_check_constraints", "can_introspect_check_constraints"
     )