I’m using Hibernate 6.2.5.Final with an entity like this:

@Entity
@Table(name = "sen_sensor_group")
public class SensorGroup implements Serializable {
  @Id
  @Column(name = "id")
  @GenericGenerator(
      name = "sen_sensor_group_id_seq",
      strategy = "enhanced-sequence",
      parameters = {
          @Parameter(name = "sequence_name", value = "sen_sensor_group_id_seq"),
          @Parameter(name = "optimizer", value = "pooled-lo"),
          @Parameter(name = "increment_size", value = "50")})
  @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "sen_sensor_group_id_seq")
  private Long id;

  @ManyToMany(fetch = FetchType.EAGER)
  @JoinTable(
      name = "sen_sensor_group_sensor",
      joinColumns = @JoinColumn(name = "sensor_group_id", nullable = false),
      inverseJoinColumns = @JoinColumn(name = "sensor_id", nullable = false)
  )
  @OrderBy("name")
  private Set<Sensor> sensors = new LinkedHashSet<>();

  // Other fields...
}

When updating the group, I get a “duplicate key value violates unique constraint” error if I reuse the set in the group when some of the items added already are part of the group. It seems like Hibernate is trying to re-insert items that had already existed in the set.

However, if I use a new set and add the already existing items the group persists correctly. Why does re-using the set cause existing items to be inserted?

For example, this does not work:

  @Transactional
  public SensorGroup updateSensorGroup(String username, SensorGroup sensorGroup) {
    var updatedSensorGroup = getSensorGroup(username, sensorGroup.getSensorGroupId());
    updatedSensorGroup.setName(sensorGroup.getName());

    // Clearing the set, then re-adding items causes constraint violation if items added
    // were already in the set previously.
    updatedSensorGroup.getSensors().clear();
    for (var sensor :sensorGroup.getSensors()) {
      var fetchedSensor = getSensor(username, sensor.getSensorId());
      updatedSensorGroup.getSensors().add(fetchedSensor);
    }

    return updatedSensorGroup;
  }

This does work as expected without error:

  @Transactional
  public SensorGroup updateSensorGroup(String username, SensorGroup sensorGroup) {
    var updatedSensorGroup = getSensorGroup(username, sensorGroup.getSensorGroupId());
    updatedSensorGroup.setName(sensorGroup.getName());

    // Using a new set and replacing the original works
    Set<Sensor> sensorsToKeep = new HashSet<>();
    for (var sensor : sensorGroup.getSensors()) {
      var fetchedSensor = getSensor(username, sensor.getSensorId());
      sensorsToKeep.add(fetchedSensor);
    }
    updatedSensorGroup.setSensors(sensorsToKeep);

    return updatedSensorGroup;
  }