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).