make change_column_comment and change_table_comment invertible

We can revert migrations using `change_column_comment` or
`change_table_comment` at current master.
However, results are not what we expect: comments are remained in new
status.
This change tells previous comment to these methods in a way like
`change_column_default`.

1 files changed, 10 insertions, 0 deletions

diff --git a/activerecord/lib/active_record/migration/compatibility.rb b/activerecord/lib/active_record/migration/compatibility.rb
index abc939826b..ff91218696 100644
--- a/activerecord/lib/active_record/migration/compatibility.rb
+++ b/activerecord/lib/active_record/migration/compatibility.rb
@@ -27,6 +27,16 @@ module ActiveRecord
           def invert_transaction(args, &block)
             [:transaction, args, block]
           end
+
+          def invert_change_column_comment(args)
+            table_name, column_name, comment = args
+            [:change_column_comment, [table_name, column_name, from: comment, to: comment]]
+          end
+
+          def invert_change_table_comment(args)
+            table_name, comment = args
+            [:change_table_comment, [table_name, from: comment, to: comment]]
+          end
         end
 
         def create_table(table_name, **options)