gistfile1.hrl

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)} || A <- mnesia:table(a)])
  ).

top_as_with_some_bs(Limit) ->
  top(
    qlc:q([ {A,bs_by_a_id(A#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.