One-To-One unidirectional relationship - jpa

Question

I have two entities, AP and APStatus, in a one-to-one unidirectional relationship. Only the AP needs to be able to access the APStatus. The only thing the APStatus needs to know is the AP's id, which will also serve as the primary key for the APStatus. Essentially, APStatus is like an embedded object, but I want a separate table for it. Here's what I have:
My entities
#Entity
public class AP {
#Id
#GeneratedValue(strategy=GenerationType.AUTO)
#Column(name="ID", nullable=false)
private int id;
#OneToOne
private APStatus apStatus;
//Getters and setters
}
#Entity
public class APStatus {
#Id
#Column(name="AP_ID", nullable=false)
private int apId;
//Getters and setters
}
My test
public class APRepositoryTest {
#Autowired
APRepository repository;
#Autowired
APStatusRepository statusRepository;
#Test
public void test() {
AP ap = new AP();
APStatus status = new APStatus();
statusRepository.save(status);
ap.setApStatus(status);
repository.save(ap);
status.setApId(ap.getId());
AP dbAp = repository.findOne(ap.getId());
assertNotNull(dbAp);
assertNotNull(dbAp.getApStatus());
assertEquals(dbAp.getId(), dbAp.getApStatus().getApId());
}
}
The assertEquals fails, expected: <1> but was <0>. And I already know why, I'm setting the status's apId field after I already saved the status and ap. But the problem with setting it before I save is that I would be setting the field equal to zero because after repository.save(ap) is executed, ap's id is auto generated to a new value (in this case 1). I've already experimented with making the relationship bidirectional and adding cascading effects but so far I've been unsuccessful. Can someone point me in the right direction or tell me how I can fix this? Thanks in advance.
EDIT: For now I'm going to make the relationship bidirectional and have the getter method in the APStatus class for the apId attribute look like the following. If someone has a better answer please share.
public int getApId() {
return ap.getId();
}

Answer 1

First, you cannot change an entities ID. It is set once and only once to identify the entity. After that, you are forcing the instance to reference something completely different, which is not supported in JPA.
That means that if the status.setApId call changes the apId field, it can only be set with the actual value.
Second, I don't quite understand the AP's apStatus reference. Does your AP have a foreign key to APStatus, or are you intending it to use the AP.ID APStatus.AP_ID relationship? My guess is that it is intended to reuse the AP.ID APStatus.AP_ID relationship as this seems more common, but only affects how you map it.
You can get this to work without changing your mappings by first saving the AP, using its ID in ApStatus and saving, then updating the relationship:
AP ap = new AP();
APStatus status = new APStatus();
repository.save(ap);
status.setApId(ap.getId());
statusRepository.save(status);
ap.setApStatus(status);
repository.save(ap);
Or you can change your mappings using JPA 2.0's derrived identities, set the cascade settings, and then only save either the the AP or the APStatus:
#Entity
public class AP {
#Id
#GeneratedValue(strategy=GenerationType.AUTO)
#Column(name="ID", nullable=false)
private int id;
#OneToOne(mappedBy = "ap", cascade=CascadeType.ALL, orphanRemoval = true)
private APStatus apStatus;
//Getters and setters
}
#Entity
public class APStatus {
#Id
#OneToOne
#JoinColumn(name="AP_ID")
private AP ap;
//Getters and setters
}
Then you can simply use:
AP ap = new AP();
APStatus status = new APStatus();
status.setAp(ap);
ap.setApStatus(status);
repository.save(ap);