显示列表中具有特定元素的所有记录

Question

How to use lists:member(User,users) in the match_object command?? I have several records in which "users" is a list contain different users , i want to display the records which contain a certain user .

find(User) ->
mnesia:transaction(fun () -> 
 case mnesia:match_object(#updt{**users==[User]**, _='_'}) of
    [Y] ->io:format("Records found for the user : ~p~n", [Y]);
    [] ->  io:format("No group found with the user : ~p ~n",[User])
end
end).`

Answer 1

Basically what you are doing in a mnesia:match_object/1 is supplying a pattern to match, no more no less. And in a pattern you can only match data and cannot specify a call to a function. As erlang does not have a patten data type we fake it by supplying a structure which is interpreted as a pattern.

Unfortunately using mnesia:select/2 will not help here. While a match spec has a "guard" you are only allowed to do typical guard things in it and calling general functions are not allowed.

Note that both mnesia:match_object/1 and mnesia:select/2 return the whole objects which match.

Answer 2

I see 2 options to solve your problem:

use

mensia:foldl(fun({updt,I,L},Acc) -> 
                case lists:member(User,L) of
                    true -> [I|Acc];
                    _ -> Acc
                end,[],Table)

or maintain a second table which records what you need ({new_table,User,[I...]})

You should choose depending on the frequency of update request, query request, the size of the table and if there are concurrent accesses or not.

Answer 3

You don't have to use match_object. I have answered a similar question:

[answered question][1] https://stackoverflow.com/a/28762454/

Basically you can query the database like you would with a SQL DB using the method i specified in the answered question. You can use lists:member/2 to find matches.