Columns reload problem when table changes

hi, all

I have a problem when I use Table.content_columns, the result returned
is just old columns, any new column which are added by add_column
would not show up. but actual content i added can be access by using
column_name eg.
@row = Table.find_by_id( id )
@field = @row.new_column_name
this is ok, but Table.content_columns which not has new_column_name
Anyone knows how I can fix this?
Thank a lot!