[erlang-questions] QLC cursor with QLC query using nested QLC.

Daniel Kwiecinski <>
Mon May 11 13:06:06 CEST 2009


The same results  with:
Erlang (SMP,ASYNC_THREADS) (BEAM) emulator version 5.7.1
stdlib-1.16.1
mnesia-4.4.9

2009/5/11 Daniel Kwiecinski <>

> Just some additional info:
>
> $ erl -version
> Erlang (SMP,ASYNC_THREADS) (BEAM) emulator version 5.6.5
> mnesia-4.4.7
> stdlib-1.15.5
>
>
> 2009/5/11 Dan Gudmundsson <>
>
> That sounds like a bug in mnesia, I will have looka at it.
>>
>> /Dan
>>
>> Daniel Kwiecinski wrote:
>>
>>> Hi,
>>>
>>>   I'm trying to implement kind of SELECT COUNT(B.ID <http://B.ID>) C,
>>> A.* FROM A, B WHERE A.ID <http://A.ID> = B.F_ID GROUP BY A.ID <
>>> http://A.ID> HAVING C > 0
>>>
>>> So I need to make join with count on details. The thing is, I want also
>>> to involve sorting and limiting with is but it seams qlc cursor don't like
>>> my
>>> approach. *as_with_bs() *works fine but* **top_as_with_some_bs(10)
>>> *returns*:**
>>>
>>> {aborted,{badarg,[{ets,first,[106291236]},
>>>                  {mnesia_tm,arrange,3},
>>>                  {mnesia_tm,t_commit,1},
>>>                  {mnesia_tm,apply_fun,3},
>>>                  {mnesia_tm,execute_transaction,5},
>>>                  {erl_eval,do_apply,5},
>>>                  {shell,exprs,6},
>>>                  {shell,eval_exprs,6}]}}*
>>>
>>> QLC with nested QLC queries works fine as long not used with cursors.
>>> Could anyone help me with it?
>>>
>>> Here is what I wrote: (http://gist.github.com/109936)
>>>
>>> *
>>> bs_by_a_id(A_id) ->
>>>  find(qlc:q([ B || B=#b{f_id=F_id} <- mnesia:table(b), F_id == A_id])).
>>>
>>> as_with_bs() ->
>>>  find(
>>>    qlc:q([ {A,bs_by_a_id(A#a.id <http://a.id>)} || A <-
>>> mnesia:table(a)])
>>>  ).
>>>
>>> top_as_with_some_bs(Limit) ->
>>>  top(
>>>    qlc:q([ {A,bs_by_a_id(A#a.id <http://a.id>)} || A <-
>>> mnesia:table(a)]),
>>>    Limit,
>>>    fun(A1,A2) -> A1 < A2  end
>>>  ).
>>>  % --- utils
>>>
>>> find(Q) ->
>>>  F = fun() -> qlc:e(Q) end,
>>>  transaction(F).
>>>
>>> % --- it returns top Limit results from query Q ordered by Order sort
>>> function
>>> top(Q, Limit, Order) ->
>>>  {atomic, Res} = mnesia:transaction(fun() ->
>>>    OQ = qlc:sort(Q, [{order,Order}]),
>>>    QC = qlc:cursor(OQ),
>>>    Res = qlc:next_answers(QC, Limit),
>>>    qlc:delete_cursor(QC),
>>>    Res
>>>  end),
>>>  Res.*
>>>
>>> Regards,
>>> Daniel
>>>
>>>
>>> ------------------------------------------------------------------------
>>>
>>> _______________________________________________
>>> erlang-questions mailing list
>>> 
>>> http://www.erlang.org/mailman/listinfo/erlang-questions
>>>
>>
>
>
> --
> Kind Regards,
> Daniel Kwiecinski
>
> m: 07952028105
> e:  
>



-- 
Kind Regards,
Daniel Kwiecinski

m: 07952028105
e:  
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