动态改变Mnesia表结构

-record(old, {key, val}).
-record(new, {key, val, extra}).
Transformer =
            fun(X) ->
                      #new{key = X#old.key,
                      val = X#old.val,
                      extra = 42}
              end,
{atomic, ok} = mnesia:transform_table(foo, Transformer,
                                                                record_info(fields, new)).