Hibernate polymorphism

Question

This is a Hibnerate polymorphism question and a data model design question; they are intertwingled. I\'ve used Hibernate in the past, and have enjoyed it, but sometimes I find

Answer 1

This is absolutely possible. You could associate the notes with the abstract Fruit class instead of repeating them in each of the implementations:

@Entity
@Inheritance
public abstract class Fruit {
   private Set notes;
   ...

   @OneToMany(cascade = CascadeType.ALL, mappedBy = "fruit")
   public Set getNotes() {
       return notes;
   }
}

@Entity
public class Apple extends Fruit {
   ...
}


@Entity
public class Orange extends Fruit {
   ...
}

@Entity
public class Note {
   private String theNote;

   @ManyToOne
   private Fruit fruit;
   ...
}

Ét voilà!

-- Addition based on comment: JPA provides multiple strategies for dealing with inheritance. The relevant section in the Java EE tutorial should help you get started.

Basically, your options are:

• Storing everything in one table and using a discriminator column to know which row is which type
• Storing each concrete class (Apple and Orange) in a separate table
• Having a common Fruit table with a discriminator column, and Apple and Orange tables with a foreign key to the Fruit table

Another edit: Noticed this is a Hibernate, not a JPA question. Does not make too much of a difference, though, as the options are the same. Here's the relevant section in the Hibernate docs.