write a Query List Comprehensions for a mnesia query

Question

I'm trying to build a small testing app with erlang+mnesia.

I have a user table build from the #user record, as defined here:

-record(user_details, {name, password}).
-record(user, {id, details}).

then I insert a user with that function:

add_sample_data() ->
    Mat = #user{
      details = #user_details{
    name = "mat", password = "mat"
       }
     }, 
    user:insert_user(Mat),

the query [U#user.details || U <- mnesia:table(user)] return a non empty list. now I'm trying to build a query which would return a list containing zero record if there is no record with details.name matching Name or the matching record if there is one.

here is the method I use (this one works):

user_exists() ->
  Fun = fun() ->
    Query = qlc:q([
      U#user.details || 
      U <- mnesia:table(user)
    ]),
    qlc:e(Query)
  end,
  case mnesia:transaction(Fun) of
    {atomic, []} -> false;
    {atomic, [_User]} -> true
  end.

I copied some stuff from this tutorial. A similar problem is solved with mnesia:select in the mne_fun_query({sport, Sport}) method (slide 19) but now I'd like to do it with qlc.

I tried various combinations but without any success (often failed at compilation time..).

I'm really new to erlang, if you can tell which query should works and explains it a little, that would be greatly appreciated!

mat.

edit

here is one version which does not work but maybe explain my problem better

user_exists() ->
  Fun = fun() ->
    Query = qlc:q([
      U#user.details || 
      U <- mnesia:table(user), 
      U#user.details.name == "mat"     <<<<< This is the line with the problem
    ]),
    qlc:e(Query)
  end,
  case mnesia:transaction(Fun) of
    {atomic, []} -> false;
    {atomic, [_User]} -> true
  end.

and the error I have:

mathieu@evangeneer:~/projects/nopair$ make
Recompile: src/resources/user_resource
src/resources/user_resource.erl:22: syntax error before: '.'
src/resources/user_resource.erl:6: function user_exists/2 undefined make:
* [erl] Error 1

Answer 1

I think the most common reason QLC code wouldn't compile is if you've left out the header file you're required to include in modules containing qlc queries. Try adding the following to your module and see if it fixes the problem:

-include_lib("stdlib/include/qlc.hrl").

Answer 2

On the problem line:

U#user.details.name == "mat"

You are attempting to access the user_details record but not naming it. Try...

(U#user.details)#user_details.name == "mat"

From my experience, the compiler doesn't figure out on its own that #user.details is a #user_details.