mnesia delete_object exception?

Question

I don't see what's wrong here, but I may just misunderstand the syntax. I'm trying to delete a "user" record from my "user" mnesia table who have the first name of "ryan" (there are multiples of them). Here is what I do:

Delete=#user{first_name = "ryan", _ = '_'},
mnesia:dirty_delete_object(Delete)

Here is my record definition:

-record(user, {id,
         username,
         password,
         email,
         first_name,
         last_name,
         last_login, % Date/time user last logged in
         reg_date}). % Date/time user registered the account

And here is the exception I'm getting:

** exception exit: {aborted,
                       {bad_type,user,
                           {user,'_','_','_','_',"ryan",'_','_','_'}}}
     in function  mnesia:abort/1
     in call from users:register/1

It seems like a perfect match. What could cause this problem?

Answer 1

mnesia:delete_object/1 and /3 are used to delete given record from the table. It does not delete multiple records based on match specification. It is just a more precise version of mnesia:delete/* - it gives you a way to delete specific record from the bag table.

To delete multiple records matching specification (e.g. with name="ryan") use the following code:

Delete=#user{first_name = "ryan", _ = '_'},
List = mnesia:dirty_match_object(Delete)
lists:foreach(fun(X) ->
                      mnesia:dirty_delete_object(X)
              end, List).

or it's transactional version:

Delete=#user{first_name = "ryan", _ = '_'},
Fun = fun() ->
              List = mnesia:match_object(Delete)
              lists:foreach(fun(X) ->
                                    mnesia:delete_object(X)
                            end, List)
      end,
mnesia:transaction(Fun).

Answer 2

it is slowness. a better way? if i want to delete age < 20, then how should i do it ?