#ElementCollection to non-collection field - jpa

Question

I have this #ElementCollection mapping so i could bring a legacy table with no unique id to work:
#Entity #Table(...)
#Inheritance(...) #DiscriminatorColumn(...)
class Notification {
#Id
#Column(name="NOTIFICATION_ID")
private BigInteger id;
}
#Entity
#DiscriminatorValue(...)
class SomeNotification extends Notification {
#ElementCollection
#CollectionTable(name="LEGACY_TABLE", joinColumns=#JoinColumn(name="NOTIFICATION_ID"))
private Set<NotificationInfo> someInformations;
}
#Embeddable
class NotificationInfo { // few columns }
I really can't touch the structure of LEGACY_TABLE, and now i am facing this:
#Entity
#DiscriminatorValue(...)
class SpecialNotification extends Notification {
// ? This is not a Collection, and it can't be a ManyToOne or OneToOne
// since there is no ID declared on NotificationInfo.
private NotificationInfo verySpecialInformation;
}
I know this is not supported by default, but i am fine to implement a Customizer to make it work with EclipseLink. The point is that for SpecialNotification instances, there will be only up to one NotificationInfo associated, instead of many, that is the case of SomeNotification.
Any thoughts about where i could start in the Customizer?
Thank you!

Answer 1

I'm not sure this will work, but it's worth a shot. Try a combination of #SecondaryTable and #AttributeOverride
#Entity
#SecondaryTable(name="LEGACY_TABLE",
pkJoinColumns=#PrimaryKeyJoinColumn(name="NOTIFICATION_ID"))
#DiscriminatorValue(...)
class SpecialNotification extends Notification {
...
#Embedded
#AttributeOverrides({
#AttributeOverride(name="someField", column=#Column(table = "LEGACY_TABLE", name="SOME_FIELD")),
#AttributeOverride(name="someOtherField", column=#Column(table = "LEGACY_TABLE", name="SOME_OTHER_FIELD"))
})
private NotificationInfo verySpecialInformation;
...
}
UPDATE
Since #SecondaryTable by default makes an inner join, which may not be desired, it can be worked around with vendor specific APIs.
If you use Hibernate (which you don't, judging by the question tags, but nevertheless), it can be done with #org.hibernate.annotations.Table, by setting optional = true.
With EclipseLink, you should make use of #DescriptorCustomizer and DescriptorQueryManager#setMultipleTableJoinExpression, you can find a (not spot-on, but close enough) code example here.