A door stay device for automatically locking a door in an open position includes a first component adapted to be attached to a stationary support, the first component including a plate having a curved cam edge with a radius matched to a swinging movement of the door between closed and open positions. The plate has a slot open at the cam edge at a location corresponding to the open position of the door. A second component is adapted to be secured at a lower or upper edge of a door, the second component having a locking member arranged to move into the slot when the locking member is aligned with the slot and the door is in the open position.