[erlang-questions] Mnesia record with composite key and QLC questions

Leon de Rooij <>
Fri May 15 10:32:12 CEST 2009

Hi all,

I want to store users in an mnesia database, but want the key not only  
to be unique on username, but also on domain.

So I defined the user record as follows:

-record(user, {domain_username, params=[]})

where domain_username = {Domain, Username}
and params = [ {Key1,Value1}, {Key2, Value2}, ... ]

I chose params to be a list because I don't know beforehand how many  
key/value tuples will be in there.

Is this the correct way to do things ? (Instead of having domain and  
username as standalone strings (lists) in the record and make it a bag  
instead of a set, or for the params having a seperate table with  
relations/foreign keys, like I would do with SQL ?)

It's also not clear to me what is the best way to search through the  

To find a user based on a param, the following function works:

Param = {"accountcode", "1234"},
Fun = fun() -> qlc:eval(qlc:q([ U || U <- mnesia:table(user),  
lists:member(Param, U#user.params) ])) end,

That works, but can this be done more efficient ? (Searching on params  
won't be done often, but still I'd like to know..)

Also, I'd like to be able to for example get a list of all users in a  
certain domain, but I can't get it to work yet..

I tried this:

Domain = "test.com",
Fun = fun() -> qlc:eval(qlc:q([ U || U <- mnesia:table(user),  
{Domain,_} =:= U#user.domain_username ])) end,

But, having an underscore for username in the tuple to match equality  
is not allowed (compilation breaks with "variable '_' is unbound").

Then I tried using assignment operator there:

Fun = fun() -> qlc:eval(qlc:q([ U || U <- mnesia:table(user),  
{Domain,_} = U#user.domain_username ])) end,

which does compile, but it breaks at runtime with:

=ERROR REPORT==== 15-May-2009::08:26:37 ===
Error in process <0.31.0> with exit value: {undef, 

** exited: {undef,[{test,get_user_by_domain,["test.com"]},
                    {shell,eval_loop,3}]} **

Can anyone tell me what is best practice in these cases ?

Thanks and kind regards,


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