Best way to print out Mnesia table

Question

I tried this code snippet:

print_next(Current) ->
    case mnesia:dirty_next(muppet, Current) of
        '$end_of_table' ->
            io:format("~n", []),
            ok;
        Next ->
            [Muppet] = mnesia:dirty_read({muppet, Next}),
            io:format("~p~n", [Muppet]),
            print_next(Next),
            ok
    end.

print() ->
    case mnesia:dirty_first(muppet) of
        '$end_of_table' ->
            ok;
        First ->
            [Muppet] = mnesia:dirty_read({muppet, First}),
            io:format("~p~n", [Muppet]),
            print_next(First),
            ok
    end.

But it is so long. Also I can use dirty_all_keys and then iterate through key list, but I want to know if there is a better way to print out Mnesia table contents.

Answer 1

Well, if the intent is to see the contents of your table, there is the application called tv, which can view both ETS and mnesia tables.

If you wish to see all the table contents on your terminal, then try something like this:

traverse_table_and_show(Table_name)->
    Iterator =  fun(Rec,_)->
                    io:format("~p~n",[Rec]),
                    []
                end,
    case mnesia:is_transaction() of
        true -> mnesia:foldl(Iterator,[],Table_name);
        false -> 
            Exec = fun({Fun,Tab}) -> mnesia:foldl(Fun, [],Tab) end,
            mnesia:activity(transaction,Exec,[{Iterator,Table_name}],mnesia_frag)
    end.

Then if your table is called muppet, you use the function as follows:

traverse_table_and_show(muppet).

Advantages of this:
If its executed within a transaction , it will have no problems of nested transactions. It is less work because its done within one mnesia transaction through mnesia iterator functionality as compared to your implementation of get_next_key -> do_read_with_key -> then read the record (these are many operations). With this, mnesia will automatically tell that it has covered all the records in your entire table. Also, if the table is fragmented, your functionality will only display records in the first fragment. This will iterate through all the fragments the belong to that table.

In this iteration mnesia method, i do nothing with the Accumulator variable which should go along with the Iterator fun and thats why you see the underscore for the second variable.

Details of this iteration can be found here: http://www.erlang.org/doc/man/mnesia.html#foldl-3