A Plane contains a Seat. This association is shown by an arrow from Plane to Seat. How do we show that a Plane contains a Seat? Do we have an attribute of type Seat in Plan class or do we have a int or String type refering to a Seat id? Basically an id will be a primary key of the Seat in DB or some unique value that can be used to get Seat details from the DB. If Plane class doesn't have an attribute of type Seat, is it still an Association, and not Dependency?