Truthteller and Liar

You played a long role-playing treasure-hunting game for several hours and finally reached end of the last level of the game. You see two doors ($A$ and $B$), and your treasure is behind one of two doors. Behind the other door is a big monster.

Beside the doors, there are two people ($a$ and $b$ respectively). Both of them know which door has the treasure, and which door has the monster. One is a truthteller (always tells the truth), and one is a liar (always lies). But you don’t know who is who. You can only ask one question to one of them.

• Your goal is to figure out correctly which door has the treasure, based on their answer. What question do you ask, and which person do you ask ?