Fixed #24245 -- Added introspection for database defaults.

Needed for tests for migrations handling of database defaults.

tests/schema/tests.py

@@ -1348,3 +1348,53 @@ class SchemaTests(TransactionTestCase):
         finally:
             # Cleanup model states
             AuthorWithM2M._meta.local_many_to_many.remove(new_field)
+
+    def test_add_field_default_dropped(self):
+        # Create the table
+        with connection.schema_editor() as editor:
+            editor.create_model(Author)
+        # Ensure there's no surname field
+        columns = self.column_classes(Author)
+        self.assertNotIn("surname", columns)
+        # Create a row
+        Author.objects.create(name='Anonymous1')
+        # Add new CharField with a default
+        new_field = CharField(max_length=15, blank=True, default='surname default')
+        new_field.set_attributes_from_name("surname")
+        with connection.schema_editor() as editor:
+            editor.add_field(Author, new_field)
+        # Ensure field was added with the right default
+        with connection.cursor() as cursor:
+            cursor.execute("SELECT surname FROM schema_author;")
+            item = cursor.fetchall()[0]
+            self.assertEqual(item[0], 'surname default')
+            # And that the default is no longer set in the database.
+            field = next(
+                f for f in connection.introspection.get_table_description(cursor, "schema_author")
+                if f.name == "surname"
+            )
+            if connection.features.can_introspect_default:
+                self.assertIsNone(field.default)
+
+    def test_alter_field_default_dropped(self):
+        # Create the table
+        with connection.schema_editor() as editor:
+            editor.create_model(Author)
+        # Create a row
+        Author.objects.create(name='Anonymous1')
+        self.assertEqual(Author.objects.get().height, None)
+        old_field = Author._meta.get_field('height')
+        # The default from the new field is used in updating existing rows.
+        new_field = IntegerField(blank=True, default=42)
+        new_field.set_attributes_from_name('height')
+        with connection.schema_editor() as editor:
+            editor.alter_field(Author, old_field, new_field)
+        self.assertEqual(Author.objects.get().height, 42)
+        # The database default should be removed.
+        with connection.cursor() as cursor:
+            field = next(
+                f for f in connection.introspection.get_table_description(cursor, "schema_author")
+                if f.name == "height"
+            )
+            if connection.features.can_introspect_default:
+                self.assertIsNone(field.default)