one-to-one

One-to-one fields are defined by the tag <one-to-one> :

<entity name="Engine">
    ...
</entity>

<entity name="Car">
    ...
    <one-to-one name="engine" ref="Engine"/>
</entity>

In this case, the object because a relation 1: 1 with an object Engine . This relationship is unidirectional in the sense that one can know the engine of a car, but nothing in the engine class allows us to know the car he belongs.

The entity Car contains a reference to Engine :

public class Car extends AuditableModel {
    ...
    @OneToOne(fetch = FetchType.LAZY, cascade = { CascadeType.PERSIST, CascadeType.MERGE })
    private Engine engine;

    public Engine getEngine() {
        return engine;
    }
    public void setEngine(Engine engine) {
        this.engine = engine;
    }
    ...
}

To create a bidirectional relationship, the slave entity must specify a return (one-to-one) and a field mappedBy attribute, which must reference the field that bears the master side relationship.

<entity name="Engine">
    ...
    <one-to-one name="car" ref="Car" mappedBy="engine"/>
</entity>

<entity name="Car">
    ...
    <one-to-one name="engine" ref="Engine"/>
</entity>