Modifying an mnesia table key

Dale Harvey <>
Fri May 29 03:35:51 CEST 2009


is the only way to modify the key of an mnesia table to copy
it to a temporary table and back again manually or am I missing
something?

-module(test).
-compile(export_all).

test() ->
    application:stop(mnesia),
    ok = mnesia:delete_schema([node()]),
    ok = mnesia:create_schema([node()]),
    application:start(mnesia),
    create(),
    mnesia:dirty_write(test, {test, "123", "some data"}),
    transform().

create() ->
    mnesia:create_table(test, [{type, bag},
                               {attributes, [id, field1]}]).

transform() ->
    F = fun({test, _Id, Field}) ->
                {test, 123, Field}
        end,
    mnesia:transform_table(test, F, [id, field1]).

>test:test().
{aborted,{"Bad transform function",test,
          #Fun<test.0.115054077>,
          {"Bad key or Record Name",
           {test,"123","some data"},
           {test,123,"some data"}}}}

(also slightly confused by the transform function in mnesia_scheme.erl that
seems to read that transforms will only work if the transformed record is
== to the original one)

transform_obj(Tab, RecName, Key, Fun, [Obj|Rest], NewArity, Type, Ws, Ds) ->
    NewObj = Fun(Obj),
    if
        size(NewObj) /= NewArity ->
            exit({"Bad arity", Obj, NewObj});
    NewObj == Obj ->
        transform_obj(Tab, RecName, Key, Fun, Rest, NewArity, Type, Ws, Ds);
        RecName == element(1, NewObj), Key == element(2, NewObj) ->
            transform_obj(Tab, RecName, Key, Fun, Rest, NewArity,
              Type, [NewObj | Ws], Ds);
    NewObj == delete ->
        case Type of
        bag -> %% Just don't write that object
           transform_obj(Tab, RecName, Key, Fun, Rest,
                 NewArity, Type, Ws, Ds);
        _ ->
            transform_obj(Tab, RecName, Key, Fun, Rest, NewArity,
                  Type, Ws, [NewObj | Ds])
        end;
        true ->
            exit({"Bad key or Record Name", Obj, NewObj})
    end;


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