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<  Advanced Erlang/OTP  ~  Mnesia QCL matching question on binaries...
seancharles
Posted: Wed Nov 28, 2007 10:25 pm Reply with quote
User Joined: 18 Jul 2007 Posts: 57
Can I improve the matching part of this query by using a bit-pattern matching rule, if so how because my poor old brain can't figure it out. I am referring to the lists:sublist blah blah,... converting binaries to strings just to compare the first part seems madness but I can't crack it (yet) with a binary match.
Code:

top(CloudName, N) when is_atom(CloudName), is_integer(N) ->
    Str = atom_to_list(CloudName),
    Len = length(Str),
    mnesia:transaction(
      fun()->
       Cursor = qlc:cursor(
                  qlc:sort(
                    %% Sort on...                                                                                               
                    qlc:q(
                      %% One day I will know how to do this 'better'!                                                           
                      [ R#tagcloud{ key = lists:nthtail( Len, binary_to_list(R#tagcloud.key))} ||
                         R <- mnesia:table(tagcloud),
                          lists:sublist( binary_to_list(R#tagcloud.key), Len) =:= Str ]),
                    %% Sort options...                                                                                         
                    [{order,descending}] )),
       qlc:next_answers( Cursor, N )
      end).

This kind of goes with my other post, http://www.trapexit.org/forum/viewtopic.php?p=34823#34823 , but since then I have read the docs *really hard* and discovered that you CAN have differently named tables with the same record structure. I refer to the Mnsesia docs where is points out that the table def 'TabDef' is what you need and that the simplistic write/1 function needs to be replaceed with write/3 where you specify the extra information.

We shall not be beaten!
Smile
Erlang rocks and Mnsesia rolls.
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