使用JPA插入记录，该记录具有一个字段作为另一个表的外键

Question

I am trying to insert data into a table and one of the fields is an FK to a different table. The problem is that using JPA and Criteria to perform the operations I would like to avoid having to fill out the entire child object to do so. Example:

@ManyToOne(fetch = FetchType.LAZY)
@JoinColumn(name="SENSOR_ID", nullable=false, updatable=false)
private Sensor sensor;

public Sensor getSensor() {
    return sensor;
}

public void setSensor(Sensor sensor) {
    this.sensor = sensor;
}

em.getTransaction().begin();
Measure m = new Measure();

m.setDeformation(measure.getDeformation());
m.setMoment(timestamp);
m.setTemperature(measure.getTemperature());
m.setTension(measure.getTension());
m.setSensorId(measure.getSensorId()); // I would like to do it like this and not having to use m.setSensor();

em.persist(m);

Answer 1

If sensor is mapped in Measure like in your example:

@ManyToOne(fetch = FetchType.LAZY)
@JoinColumn(name="SENSOR_ID", nullable=false, updatable=false)
private Sensor sensor;

You need to fill sensor instance. Sensor is not saved with measure because by default no operations are cascaded. It must be saved before calling this.

To not fill whole sensor, use sensor_id FK instead of Sensor in Measure class.

@Column(nullable = true)
private Long sensor_id;

To use both one must be read only:

@ManyToOne(fetch = FetchType.LAZY)
@JoinColumn(name="SENSOR_ID", updatable=false, insertable = false)
private Sensor sensor;