JPA ManyToMany unidirectional relationship

Question

Lets say we have two Entities, Entity Node and Entity Cluster. A Cluster has many Nodes. A Node can belong to multiple Cluster. So in Cluster there is a @ManyToMany annotation. However Node is unaware of any Cluster it belongs to (intentional).

When I delete a Cluster I want to keep all the Nodes (in future I may add those Nodes to another Cluster) it had. So I don't need Cascade.DELETE. But If I delete a Node, all the Cluster it belonged to should be updated (the Node will be removed from them).

What is the best way to handle this using JPA ?

Answer 1

I am way too late to this thread, but I had similar issues with Spring Boot 2.4. I had 2 ManyToMany unidirectional relationships from the parent entity to the same child entity.

I would end up getting 1 or 2 less entity relationships saved in the join table.

Then I realized that I had missed out on the most basic thing for an entity/pojo - overriding the hashCode and equals method!

Once I added those, Hibernate started to behave as expected.

Answer 2

In fact in a unidirectional relationship, you can specify the holder entity of the relationship using joinColumn="clusterId" property in the @ManyToMany annotation in the `Cluster class.

This is how should be your code:

@Entity
@Table(name="Cluster")
public class Cluster {

    //Id and properties

    @ManyToMany(cascade = {CascadeType.ALL})
    @JoinTable(name="Cluster_Nodes", 
                joinColumns={@JoinColumn(name="clusterId")}, 
                inverseJoinColumns={@JoinColumn(name="nodeId")})
    private Set<Node> nodes = new HashSet<Node>();

    // Getter and Setter methods
}

For further reading please take a look at :

Hibernate @ManyToMany Unidirectional and Bidirectional