the straight and narrow

If someone keeps you on the straight and narrow, they help you to live an honest, decent life and prevent you from doing immoral or illegal things.

He depended on me when he was working to keep him on the straight and narrow, to keep some sense of perspective about what life was all about.

Education a devout Catholic, is determined to introduce a new classroom culture of morality to set youngsters on the straight and narrow.

The goal is to prevent them from straying from the straight and narrow.

straight as a die¹    

If you say that someone is straight as a die, you mean that they are completely honest. This expression is used in British English.

But I got the impression that deviousness is not one of characteristics. He is, as the know more about deviousness and deception than any other race in the would say, as straight as a die.

straight as a die²    

If you say that something is straight as a die, you are emphasizing that it is very straight. This expression is used in British English.

The streets are lined up, straight as a die, along the left bank of the Guadiana estuary.

He pauses to point out a trunk that rises straight as a die -- an ash tree.