Erlang Mnesia Unique keys except primariy key

Question

I want to create the following table in mnesia:

-record(cuser, {
id,
login_id,
email,
....}).

where login_id and email are both unique in business logic.

How can I implement it in mnesia?

Thanks in advance.

Answer 1

The simplest way is to performs all the tests in a single mnesia transaction. the following code does it.

Be aware that this code does not check if the Id already exists, so if you try to add a new user with existing id and new login id and email, it will simply overwrite the existing record (that is the behavior of set).

%% record declaration
-record (cuser, {id,login_id,email}),
%% create a table for test
mnesia:create_schema([node()]),
application:start(mnesia),
mnesia:create_table(cuser,[{attributes,record_info(fields,cuser)},
                           {index,[#cuser.login_id,#cuser.email]},
                           {disc_copies,[node()]},
                           {type,set}]),
%% write 2 record for test
Wr = fun(I,L,E) -> mnesia:write(#cuser{id=I,login_id=L,email=E}) end,
mnesia:transaction(Wr,[u1,l1,m1]),
mnesia:transaction(Wr,[u2,l2,m2]),
%% create a function for filtering
M = fun(Nl,Ne) -> 
        Match = ets:fun2ms(fun(#cuser{id=I,login_id=Li,email=E}) when Li =:= Nl; E =:= Ne -> {I,Li,E} end),
        mnesia:select(cuser,Match)
    end,
%% create a function to add a new user
Add = fun(I,Nl,Ne) ->
        {atomic,R} = mnesia:transaction(M,[Nl,Ne]),
        case R of
          [] -> mnesia:transaction(Wr,[I,Nl,Ne]);
          R -> {already_exist,R}
        end
       end,
%% add new user
mnesia:transaction(Add,[i3,l3,m1]), %% will return {atomic,{already_exist,[{u1,l1,m1}]}}
mnesia:transaction(Add,[i3,l3,m3]), %% will return {atomic,{atomic,ok}}

To check if the Id already exists simply add the test in the filter function:

%% create a function for filtering
M = fun(Ni,Nl,Ne) -> 
        Match = ets:fun2ms(fun(#cuser{id=I,login_id=Li,email=E}) when I =:= Ni; Li =:= Nl; E =:= Ne -> {I,Li,E} end),
        mnesia:select(cuser,Match)
    end,
%% create a function to add a new user
Add = fun(I,Nl,Ne) ->
        {atomic,R} = mnesia:transaction(M,[I,Nl,Ne]),
        case R of
          [] -> mnesia:transaction(Wr,[I,Nl,Ne]);
          R -> {already_exist,R}
        end
       end,

Answer 2

Mneisa is a key-value storage system, which only guarantees the key is unique.

The only way I can come up with is to use a tuple to save {id, login_id} as the primary key.

Unfortunately, this can only guarantee one of them is unique. However, at least you can query an item with {id, login_id}.

For example:

-record(cuser, {
    uid, %uid = {id, login_id}
    email,
    ....}).