Spring Data JPA获取实体外键而不会导致从属实体延迟加载

Question

I have an @Entity A that references another entity B using OneToOne relation ship. I fetch entity A using spring data JpaRepository

A a = aRepository.findById(1);
int b_id = a.getB().getId();

As you can see I need to query ID of the B table, however in order to do that, I need to call getter of the B table, which will cause lazy-loading the B table itself. I do not want to do that because the only thing I need is the get ID, nothing else, and that ID is present in the first A table.

Is there any trick that will help me to get ID of the dependent table without triggering new query?

UPDATE

@Entity
class A {
    @Id
    private Long id;

    @OneToOne
    private B b;
}

@Entity
class {
    @Id
    private Long id;
}

Answer 1

Without looking at the entity mapping, I suspect, your entity classes might be using hibernate annotations on the field . With this if you call even the getId() method as in a.getB().getId() on the entity it will result in initializing the proxy (ie, B object) and hits the database to fetch it.

So if the intent is only to get the id of the entity you can place the hibernate annotations on the getter methods instead. This doesn't result initializing the proxy (B object) to return the id . Although accessing any property other than id will result in hitting the database.

Have a look at related bug at HHH-3718

So, try using property/getter AccessType instead of field access. As an example instead of placing the annotations on field

@Id
@GeneratedValue(...)
private long id;

place them on the getters

@Id
@GeneratedValue(...)
public long getId() { ... }

Make sure you make similar changes to all the fields of B entity. Although you can explore @Access(AccessType.PROPERTY/FIELD) later.

There is already a related bug HHH-3718 regarding this behavior.

And a related topic on hibernate forum regarding field vs property access type that might be of interest for you Field Vs Property access

Posting your entities classes would help, if this doesn't resolve the issue.