Hibernate Onetomany self join adds unique constraint

Question

I would like to add a one to many relationship of an entity with itself:

@Entity
public class Layer {

@Id
private Long id;

@Column(name = "PARENT_ID")
private String parentId;

@OneToMany
@JoinColumn(name = "PARENT_ID", referencedColumnName = "PARENT_ID", updatable = false, insertable = false, foreignKey = @ForeignKey(ConstraintMode.NO_CONSTRAINT))
private Set<Layer> siblings;    
}

This means that I want to get all the other entities sharing the same parentId. The reason I want it is because it would be easier to write queries for this entity.

The problem is that on database generation(spring.jpa.properties.hibernate.hbm2ddl.auto=create) it also adds a unique constraint on the PARENT_ID column and I do not want that.

How do I keep hibernate from adding the unique constraint?

Answer 1

Your mapping currently makes children reference their parent by the column PARENT_ID. In order to use something as a reference it has to be unique.

What you probably want to have is the children to reference their parent by its id.

This should do the trick:

@OneToMany
@JoinColumn(name = "PARENT_ID", referencedColumnName = "ID", updatable = false, insertable = false, foreignKey = @ForeignKey(ConstraintMode.NO_CONSTRAINT))
private Set<Layer> children;    

I'm not sure that you can have a String parentId as well. First it maps to the same column as the children property, but with a different type: id is of type Long, while parentId which should reference it is of type String.

The normal way to do this would be to make a property Layer parent which @ManyToOne and mappedBy children.