make change_column_null reversible. Closes #13576.

Closes #13623.

activerecord/CHANGELOG.md

@@ -1,3 +1,7 @@
+*   Make `change_column_null` revertable. Fixes #13576.
+
+    *Yves Senn*, *Nishant Modak*, *Prathamesh Sonpatki*
+
 *   Don't create/drop the test database if RAILS_ENV is specified explicitely.
 
     Previously, when the environment was development, we would always

activerecord/lib/active_record/migration/command_recorder.rb

@@ -74,7 +74,7 @@ module ActiveRecord
         :rename_index, :rename_column, :add_index, :remove_index, :add_timestamps, :remove_timestamps,
         :change_column_default, :add_reference, :remove_reference, :transaction,
         :drop_join_table, :drop_table, :execute_block, :enable_extension,
-        :change_column, :execute, :remove_columns, # irreversible methods need to be here too
+        :change_column, :execute, :remove_columns, :change_column_null # irreversible methods need to be here too
       ].each do |method|
         class_eval <<-EOV, __FILE__, __LINE__ + 1
           def #{method}(*args, &block)          # def create_table(*args, &block)
@@ -157,6 +157,11 @@ module ActiveRecord
       alias :invert_add_belongs_to :invert_add_reference
       alias :invert_remove_belongs_to :invert_remove_reference
 
+      def invert_change_column_null(args)
+        args[2] = !args[2]
+        [:change_column_null, args]
+      end
+
       # Forwards any missing method call to the \target.
       def method_missing(method, *args, &block)
         if @delegate.respond_to?(method)

activerecord/test/cases/migration/change_schema_test.rb

@@ -308,6 +308,22 @@ module ActiveRecord
         assert_equal 2000, connection.select_values("SELECT money FROM testings").first.to_i
       end
 
+      def test_change_column_null
+        testing_table_with_only_foo_attribute do
+          notnull_migration = Class.new(ActiveRecord::Migration) do
+            def change
+              change_column_null :testings, :foo, false
+            end
+          end
+          notnull_migration.new.suppress_messages do
+            notnull_migration.migrate(:up)
+            assert_equal false, connection.columns(:testings).find{ |c| c.name == "foo"}.null
+            notnull_migration.migrate(:down)
+            assert connection.columns(:testings).find{ |c| c.name == "foo"}.null
+          end
+        end
+      end
+
       def test_column_exists
         connection.create_table :testings do |t|
           t.column :foo, :string