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<  Erlang  ~  Update "query" for erlang?
laserlars
Posted: Tue Nov 27, 2007 2:19 pm Reply with quote
Joined: 10 Sep 2007 Posts: 5
Hello

Someone who know what a update query for erlang would look like?

Or do I need to make a write on a existing key to update it? Apparently if i do, i need to give it all its fields again or they would be set to unassigned?
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francesco
Posted: Fri Nov 30, 2007 6:37 am Reply with quote
User Joined: 07 Jul 2006 Posts: 249 Location: London
I assume you are talking about overwriting entries in destructive tables such as mnesia, ets and dets? If so, then yes, you need to overwrite the existing record / tuple. It will work with the exception of tables of type bags and duplicate bags, where you first need to delete the element.

I hope the above helps. Let me know if you need an example.

Francesco
--
http://www.erlang-consulting.com
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laserlars
Posted: Fri Nov 30, 2007 7:56 pm Reply with quote
Joined: 10 Sep 2007 Posts: 5
Well, I got a solution running, but I must say that it is quite ugly, now i have to retreive all the fields from the row and then make a write on the same key with all the fields again.

Code:

changeArea(UserId, Area) ->   
   [{GroupId,Name, Haircut, Shirt, Pants, Position, Level, Node, Socket, Vote}] = database:do(qlc:q([{X#active_connections.groupid, X#active_connections.name, X#active_connections.haircut, X#active_connections.shirt, X#active_connections.pants, X#active_connections.position, X#active_connections.level, X#active_connections.node, X#active_connections.socket, X#active_connections.vote} || X <- mnesia:table(active_connections),
            X#active_connections.userid =:= UserId])),
   
   Row = #active_connections{userid = UserId, groupid=GroupId, name=Name, haircut=Haircut, shirt=Shirt, pants=Pants, position=Position, node = Node, socket = Socket, level = Level,area = Area, vote=Vote},
   F = fun() ->
      mnesia:write(Row)
   end,
   mnesia:transaction(F).


I don't think you will get any girls with such ugly code Smile If you have a better suggestion please shoot!
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seancharles
Posted: Fri Nov 30, 2007 9:40 pm Reply with quote
User Joined: 18 Jul 2007 Posts: 57
I am no mnesia expert but I think that this:
Code:

changeArea(UserId, Area) ->   
   [{GroupId,Name, Haircut, Shirt, Pants, Position, Level, Node, Socket, Vote}] = database:do(qlc:q([{X#active_connections.groupid, X#active_connections.name, X#active_connections.haircut, X#active_connections.shirt, X#active_connections.pants, X#active_connections.position, X#active_connections.level, X#active_connections.node, X#active_connections.socket, X#active_connections.vote} || X <- mnesia:table(active_connections),
            X#active_connections.userid =:= UserId])),
   
   Row = #active_connections{userid = UserId, groupid=GroupId, name=Name, haircut=Haircut, shirt=Shirt, pants=Pants, position=Position, node = Node, socket = Socket, level = Level,area = Area, vote=Vote},
   F = fun() ->
      mnesia:write(Row)
   end,
   mnesia:transaction(F).

could be replaced with this, these are the ways that I use Erlang / mnesia and it works for me.
Code:

changeArea(UserId, Area) ->   
   %% Assumes 1 match returned from the query
   Match = hd( database:do(
      qlc:q([ X || X <- mnesia:table(active_connections),
                   X#active_connections.userid =:= UserId]))),

   %% Modify just the area of the matching record
   Row = Match#active_connections{area = Area},

   %% Update the table with the modified record
   mnesia:transaction( fun() -> mnesia:write(Row) end ).

That's off the top of my now Erlang-battered head, without typing it in anywhere other than here but I am reasonably confident it's correct!

Hope that helped !?!?
Smile
->Sean.
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laserlars
Posted: Tue Dec 04, 2007 4:01 pm Reply with quote
Joined: 10 Sep 2007 Posts: 5
Worked like a charm!

Thanks alot
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seancharles
Posted: Tue Dec 04, 2007 4:54 pm Reply with quote
User Joined: 18 Jul 2007 Posts: 57
I am as pleased as you are!
Smile
->Sean
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