JPA: implicit cascades for relationships mapped as @ManyToMany @JoinTable?

Question

I have the following mapping:

@Entity
@Table(name = "Prequalifications")
public class Prequalification implements Serializable
{
    ...

    @ManyToMany
    @JoinTable(name = "Partnerships", joinColumns = @JoinColumn(name = "prequalification_id", referencedColumnName = "id"), inverseJoinColumns = @JoinColumn(name = "company_id", referencedColumnName = "id"))
    private Set<Company> companies;

    ...
}

In a @ManyToMany + @JoinTable mapped relationship, isn't it kind of implicit that the association (link) entities (here Partnerships) are automatically persisted, removed, etc. even though

by default, relationships have an empty cascade set

? The above quote was taken from "Pro JPA 2, by Mike Keith".

Executing

em.merge(prequalification);

on the above entity does persist the associated partnerships without any cascade types specified.

Am I correct that this implicit cascade has to be performed? This isn't mentioned anywhere I looked...

I am assuming you are familiar with Hibernate at some point, and you can refer Hibernate and Hibernate annotation documents for your question. They have chapters very detailed on relationship definition and cascade operations. — Jerry Tian, Mar 22 '12 at 04:10

score 15 · Accepted Answer · answered Mar 22 '12 at 13:00

The rows in the join table will be inserted/deleted as part of the owning Entity (if bi-directional the side without the mappedBy). So if you persist or remove or update the Prequalification the join table rows will also be inserted or deleted.

The target Company objects will not be cascaded to. So on remove() they will not be deleted, if the list is updated they will not be deleted unless orphanRemovla is set. Persist should also not be cascaded, but what happens when you have references to "detached" objects is somewhat of a grey area. Technically an error should be thrown, because the object is new and the relationship was not cascade persist. It may also try to insert and get a constraint error. It should not cascade the persist, although your object model is technically in an invalid state, so what occurs may depend on the provider.

@James It's not clear to me where in the JPA spec. (unless you are referring to a Hibernate spec) it states (explicitly or to the effect) 'rows in the join table will be inserted as part of the owning entity'. The JPA spec. 11.1.30 for example states 'The cascade element specifies the set of cascadable operations that are propagated to the associated entity.' It doesn't mention of join records in the absence of a cascade element and what happens to them. Have you another reference? — D-Dᴙum, Jul 17 '18 at 22:22

Joao Monteiro · Answer 2 · 2023-02-20T14:06:02.960

Wanted to add a comment, but don't have enough rep for it.

I had the same question as @D-Dᴙum: "Where in the docs can we find a reference to this behaviour?"

I found it in the Hibernate docs (many-to-many).
If you scroll just a bit just below the code example there, you will find:

When an entity is removed from the @ManyToMany collection, Hibernate simply deletes the joining record in the link table. Unfortunately, this operation requires removing all entries associated with a given parent and recreating the ones that are listed in the current running persistent context.

Where the "link table" refers to the "join table".

Hope this helps.

JPA: implicit cascades for relationships mapped as @ManyToMany @JoinTable?

2 Answers2