In other words: an airline flight, for example from Amsterdam to Paris, can only be deleted by a user if there are no passengers assigned to that flight. In other other words, if a plane flies from New Delhi to Mumbai, it should only be deleted (cancelled) if no passengers will fly on the plane.
Hope this helps!
Depending on your domain model, if you have an entity for Flight and have it associated with an entity Passenger, then you can set the delete-behavior of that association to only allow a flight to be deleted if no passenger is associated with it.
You can double click the association and you can configure on delete options, I think.
You should set a validation to delete action (or change it if already exixts) like this : not(flight_passenger/passenger). This means that there is no object (passenger) that hangt the association between flight and passenger.