For that, I make the OrderID an internal column the users never see. This way, they'll never update it. Then, for the foreign key, specify Enforce Foreign Key Constraints and Cascade for the delete rule.
This way, the database will forbid an Order Detail that's not connected to an Order, and if an order is deleted, the details will go away too.
Sometimes, you want to restrict a delete, like, don't delete a customer that has orders, but the foreign key constraint doesn't do that for you.
You just can't do this stuff unless you're at least half a bubble off center.[/font]