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, 4 insertions, 2 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb b/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
index 2a2b234ef8..8b907759c6 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
@@ -285,7 +285,8 @@ module ActiveRecord
         SQL
       end
 
-      def change_table_comment(table_name, comment) #:nodoc:
+      def change_table_comment(table_name, comment_or_changes) # :nodoc:
+        comment = extract_new_comment_value(comment_or_changes)
         comment = "" if comment.nil?
         execute("ALTER TABLE #{quote_table_name(table_name)} COMMENT #{quote(comment)}")
       end
@@ -341,7 +342,8 @@ module ActiveRecord
         change_column table_name, column_name, nil, null: null
       end
 
-      def change_column_comment(table_name, column_name, comment) #:nodoc:
+      def change_column_comment(table_name, column_name, comment_or_changes) # :nodoc:
+        comment = extract_new_comment_value(comment_or_changes)
         change_column table_name, column_name, nil, comment: comment
       end