Question

I have a scenario where I have a entity hierarchy structure implemented in hibernate.
It is InheritanceType.JOINED

Parent class:

@Entity 
@Table(name = "LOY") 
@Inheritance(strategy=InheritanceType.JOINED)
public class Loy implements Serializable 

Child class boundary

@Entity 
@Table(name = "LOY_BOUNDARY") 
@PrimaryKeyJoinColumn(name="ID")
public class LoyBoundary implements Serializable 

Child class percentage

@Entity 
@Table(name = "LOY_PERCENTAGE") 
@PrimaryKeyJoinColumn(name="ID")
public class LoyPercentage implements Serializable 

I have a Customer entity class which is linked to the Loy entity class in a @ManyToOne.
The Customer can only be linked to one Loy at a time.

What I want to achieve is that I want to query the Customer class with unique id (passport number) and then get The Loy for the specific Customer through the @ManyToOne mapping in the Customer entity.

The problem that I’m sitting with is that I do not know which subclass of Loy is linked to the Customer.

1. I can go instanceOf to get the specific subclass but I want to try and avoid it.
2. I can also add a visitor pattern in the subclass entity, but not sure if this is best practice.

Answer 1

I would go with a visitor. Using instanceof won’t work if the ManyToOne is lazy-loaded, because the actual type of the Loy won’t be any of your subclasses, but a Hibernate proxy extending the Loy class. Indeed, Hibernate has no way to know, from the ID of the loy in the customer, which kind of Loy it refers to.

And a visitor is more OO and cleaner anyway.