Inheritance Mapping and Relationships

[Brian Smith]

I am a little confused when it comes to mapping inheritance in JPA (Hibernate Annotations)

I have an entity (AddressLookup) which has a subclass (Landmark). The parent entity is related to another entity (ServiceRequest) OneToMany.

I am using the Table per subclass (JOINED) mapping pattern.

AddressLookup

@Entity
@Inheritance(strategy=InheritanceType.JOINED)
@Table(name = "ADDRESS_LOOKUP")
@DiscriminatorColumn(name="ADDRESS_TYPE",discriminatorType=DiscriminatorType.STRING)
@DiscriminatorValue("ADDRESS")
 
public class AddressLookup implements Serializable {
 
    @Id
    @Column(name = "ADDRESS_LOOKUP_ID", nullable = false)        
    private Long addressLookupId;
 
    @OneToMany(cascade = CascadeType.ALL, mappedBy="addressLookup")
    private Collection<ServiceRequest> serviceRequestCollection;
...

Landmark

@Entity
@PrimaryKeyJoinColumn(name="LANDMARK_ID")
@Table(name = "LANDMARK")
@DiscriminatorValue("LANDMARK")
public class Landmark extends AddressLookup implements Serializable {
    @Column(name = "LANDMARK_ID", nullable = false, insertable=false, updatable=false)
    private Long landmarkId
...

ServiceRequest

@Entity
@Table(name = "SERVICE_REQUEST")
public class ServiceRequest implements Serializable {
 
    @Id
    @Column(name = "REQUEST_ID", nullable = false)
    private Long requestId;
 
    @ManyToOne
    @JoinColumn(name="ADDRESS_LOOKUP_ID", referencedColumnName="ADDRESS_LOOKUP_ID")
    private AddressLookup addressLookup;
...

Now when writing a query how do I refer to Landmarks via ServiceRequests?

In order for the following to be valid, should I map Landmark to AddressLookup OneToOne?

Landmark landmark = serviceRequest.getAddressLookup.getLandmark();

It is not a problem going the other way since Landmark inherits getServiceRequestCollection from AddressLookup but ServiceRequest knows nothing of the subclass Landmark.

Thanks for any help.

-Brian

[Shivani Chandna]

**<Newbie here>

Hello Brian,

ServiceRequest can continue to keep the AddressLookup as the reference point.

Once we get all the AddressLookups linked to ServiceRequest we can keep a method inside AddressLookup which returns the complete

We can keep a method inside the AddressLookup that gets all the Landmarks or any other child classes which may be linked to ServiceRequest. The only issue is the signature of the method :

public AddressLookup getAddressLookup(ServiceRequest sr)
{
//fire query where you retrieve address lookup and the associated extra
information within any child class.
// i.e get landmarks linked through join between AddressLookup and
Landmark (Join being defined through the Id)
// This way if tomorrow some ServiceRequest are linked to Landmark
and others linked to NewlyDefinedLandmarks(Another table extending
AddressLookup) there would be no impact on code.
}

But how can ServiceRequest access the properties specific to Landmark or NewlyDefinedLandmark ?. Casting?.

Regards,
Shivani