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/postgresql/schema_statements.rb b/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb
index ec8af73d07..40c5e51d92 100644
--- a/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb
+++ b/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb
@@ -418,14 +418,16 @@ module ActiveRecord
         end
 
         # Adds comment for given table column or drops it if +comment+ is a +nil+
-        def change_column_comment(table_name, column_name, comment) # :nodoc:
+        def change_column_comment(table_name, column_name, comment_or_changes) # :nodoc:
           clear_cache!
+          comment = extract_new_comment_value(comment_or_changes)
           execute "COMMENT ON COLUMN #{quote_table_name(table_name)}.#{quote_column_name(column_name)} IS #{quote(comment)}"
         end
 
         # Adds comment for given table or drops it if +comment+ is a +nil+
-        def change_table_comment(table_name, comment) # :nodoc:
+        def change_table_comment(table_name, comment_or_changes) # :nodoc:
           clear_cache!
+          comment = extract_new_comment_value(comment_or_changes)
           execute "COMMENT ON TABLE #{quote_table_name(table_name)} IS #{quote(comment)}"
         end