一个映射实体中的外键为空

Question

I was wondering why when I mapping two entity with mappedBy , in target class I have ID my member class, but in my member class I don't have target class ID . Example code:

 //class Onion
@OneToOne(mappedBy = "onion", fetch = FetchType.LAZY, cascade = CascadeType.ALL)
private Potatoe potatoe;

//class Potatoe
@OneToOne(cascade = CascadeType.ALL)
private Onion onion;

And everything works fine, I have a bidirectional relationship. But on DB I noticed that class Onion doesn't has ID from Potatoe , but Potatoe has Onion ID . I know that when I want take potatoe object from onion object, persistence querying database for potatoe which has onion ID.

Why is it looking that? Is it possible to have potatoe ID in onion table?

Eclipslink 2.5.0 PostgreSQL 9.1

Answer 1

It looks this way, with only a single foreign key because you have only a single (bidirectional) relationship controlled by the Potatoe->Onion mapping, so it is represented with the Potatoe table having a foreign key to Onion. If you want another foreign key from Onion to potatoe, it would mean you are asking for a relationship that can be entirely independent of the Potatoe->Onion relationship. This is easily done by just removing the mappedBy designator, which would make the Onion->Potatoe mapping completely independent and use its own foriegn key. Your model then would have two unidirectional mappings that must both be maintained.