find_mob(MobRef) ->
    FindRef = fun(Elem) -> Elem#mob.ref =:= MobRef end,
    Q = qlc:q([X ||
            X = #world{stuff=Stuff} <- mnesia:table(world),
            case proplists:lookup_all(mob, Stuff) of
                [] -> false;
                List -> lists:any(FindRef, List)
            end]),
    F = fun() -> qlc:eval(Q) end,
    case mnesia:transaction(F) of
        {atomic, [Square]} -> Square;
        _ -> nil
    end.