JPA + Hibernate - How to get FK of child entity without fetching that entity?

Question

A possible answer to my question is located here: How can I retrieve the foreign key from a JPA ManyToOne mapping without hitting the target table?

However, the preferable solution (property access) does not work in my case (I got missing column exception - why?)

The model looks like this: Entities Parent and Child. Table parent has column child_id which is PK of child table so it is typical @ManyToOne relation.

Now the point is, if I fetch Parent entities, I need to have access to FK value (aka. PK of Child entity) without fetching Child entities. How can I do that?

I use Annotations and my mapping looks as follows:

@Entity
@Table(name = "parent")
public class Parent extends AbstractEntity {

@ManyToOne(fetch=FetchType.LAZY)
@JoinColumn(name = "patent_id", nullable = true)
private Child child;

@Column(name="child_id",insertable=false,updatable=false)
private Integer childId;

public Child getChild() {
    return patent;
}

public void setChild(Child child) {
    this.child = child;
}


public Integer getChildId(){
    return childId;
}
}

And what I want to do is call parent.getChild().getId() without extra fetches of Child entity from DB.

According to the answer I have mentioned above, If I moved annotations from field to getter (in Parent entity am I right?), requested behavior would be out of the box. However, when I move annotations to getter, I get a validation exception that child column is missing (curious, why child not child_id as declared?)

PS: Shown workaround to declare a FK column as separate field works fine, but I don't think that this is the way it should be done.

I think when you do a lazy load hibernate do not even fetch that column. So if you want that to appear without fetching you should use native query. — dinesh707, Sep 17 '14 at 08:06
Try @Formula annotation and type out what you want. might work. Not tested — dinesh707, Sep 17 '14 at 08:14
org.hibernate.annotations.Formula; http://stackoverflow.com/questions/2986318/calculated-property-with-jpa-hibernate — dinesh707, Sep 17 '14 at 08:16
@dinesh Please stick to JPA standard. Its not `JPA` annotation. — Antoniossss, Sep 17 '14 at 08:17

score 0 · Accepted Answer · edited Jul 11 '16 at 16:26

0

OK, after reading following article http://256stuff.com/gray/docs/misc/hibernate_lazy_field_access_annotations.shtml I have realized, that property access should be to the property I want to fetch, not the actual child object. So changing id access of AbstractEntityfrom field to property, makes the trick.

edited Jul 11 '16 at 16:26

Gray

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answered Sep 17 '14 at 08:24

Antoniossss

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JPA + Hibernate - How to get FK of child entity without fetching that entity?

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