Is it necessary to fetch an entity in order to reference it, using JPA and MySQL?

Question

I'm having an application that creates lots of rows which reference two other entities, i.e. there are two foreign key references in the row which realize ManyToOne relationships. These are the two entities being referenced: ``` CREATE TABLE a ( `id` INT NOT NULL auto_increment, -- lots of other attributes, PRIMARY KEY (id) ) CREATE TABLE b ( `id` INT NOT NULL auto_increment, -- lots of other attributes, PRIMARY KEY (id) ) ``` This is the entity which references a and b: ``` CREATE TABLE x ( `id` INT NOT NULL auto_increment, `f_a` INT NOT NULL, `f_b` INT NOT NULL, CONSTRAINT `FK_a` FOREIGN KEY (`f_a`) REFERENCES `a` (`id`), CONSTRAINT `FK_b` FOREIGN KEY (`f_b`) REFERENCES `b` (`id`), ) ``` a, b, and x are mapped to the classes A, B, and X using JPA via EclipseLink, it's pretty basic and there is nothing fancy about it. When I create an X in Java I do the following (note that a and b already exist and I do know their IDs only - they are feeded from a legacy application): ``` A a = enti…

Accepted Answer

You can get a "reference" instance using EntityManager's `getReference()` method rather than loading the entity using find(). These references are lazily fetched, so in your case there would be no need to fetch them at all.

Your code would look similar:

```
A a = entityManager.getReference(A.class, knownIdForA);
B b = entityManager.getReference(B.class, knownIdForB);

X x = new X();
x.setA(a);
x.setB(b);

entityManager.persist(x);
```

but only the persist() call would access your DB.

Is it necessary to fetch an entity in order to reference it, using JPA and MySQL?

Solution

Problem