Changing column null does not change default function

activerecord/CHANGELOG.md

@@ -1,3 +1,7 @@
+*   Fix MySQL default functions getting dropped when changing a column's nullability.
+
+    *Bastian Bartmann*
+
 *   SQLite extensions can be configured in `config/database.yml`.
 
     The database configuration option `extensions:` allows an application to load SQLite extensions

activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb

@@ -403,7 +403,11 @@ def build_change_column_definition(table_name, column_name, type, **options) # :
         type ||= column.sql_type
 
         unless options.key?(:default)
-          options[:default] = column.default
+          options[:default] = if column.default_function
+            -> { column.default_function }
+          else
+            column.default
+          end
         end
 
         unless options.key?(:null)

activerecord/test/cases/migration/columns_test.rb

@@ -346,6 +346,20 @@ def test_change_column_null_with_non_boolean_arguments_raises
         assert_equal "change_column_null expects a boolean value (true for NULL, false for NOT NULL). Got: #{{ from: true, to: false }}", e.message
       end
 
+      def test_change_column_null_does_not_change_default_functions
+        skip unless current_adapter?(:Mysql2Adapter, :TrilogyAdapter) && supports_default_expression?
+
+        function = connection.mariadb? ? "current_timestamp(6)" : "(now())"
+
+        connection.change_column_default "test_models", "created_at", -> { function }
+        TestModel.reset_column_information
+        assert_equal function, TestModel.columns_hash["created_at"].default_function
+
+        connection.change_column_null "test_models", "created_at", true
+        TestModel.reset_column_information
+        assert_equal function, TestModel.columns_hash["created_at"].default_function
+      end
+
       def test_remove_column_no_second_parameter_raises_exception
         assert_raise(ArgumentError) { connection.remove_column("funny") }
       end