[erlang-questions] mnesia: composite keys?

Viktor Sovietov <>
Sat Jan 10 00:04:50 CET 2009


If you use ordered_set tables that tuple keys are useful, because you
can organize hierarchies inside one table. In other hand, you can't
save such tables on disk, so... Anyway, Erlang isn't MUMPS, so you
simply have not any mechanism to operate with complex keys.

Sincerely,

--Victor

On Jan 9, 11:45 pm, Peter Sabaini <> wrote:
> Hello list,
>
> I've read[1] that mnesia doesn't support composite keys. It seems I can use
> keys made up of tuples though:
>
> ()11> mnesia:create_table(test, [{attributes, [id, data]}]).
> {atomic,ok}
> ()19> Ins = fun() -> mnesia:write({test, {a_key, 0}, "asdfasdf"})
> end.
> #Fun<erl_eval.20.67289768>
> ()20> mnesia:transaction(Ins).
> {atomic,ok}
> ()21> R = fun() -> mnesia:read(test, {a_key, 0}) end.
> #Fun<erl_eval.20.67289768>
> ()22> mnesia:transaction(R).
> {atomic,[{test,{a_key,0},"asdfasdf"}]}
>
> Is tuples as keys common practice or A Really Bad Idea(tm)?
>
> thx,
> peter.
>
> [1]http://www.erlang.org/pipermail/erlang-questions/2008-February/032903...
>
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