add mappedBy to @ManyToOne and @OneToOne

The couple of weeks I've been thinking about the following sort of code, which is already possible in JPA:

@Entity
class Child {
     @Id 
     Long id;

     Long parentId;

     @ManyToOne 
     @Column(name="parentId", 
             insertable=false, updatable=false)
     Parent parent;

     ...
}

Here the parentId field and the parent association are mapped to the same column.

It seems to me that this sort of thing is not uncommon, but unfortunately that @Column annotation is quite clumsy.

We could make it quite a lot more elegant by allowing the following:

@Entity
class Child {
     @Id 
     Long id;

     Long parentId;

     @ManyToOne(mappedBy=Child.PARENT_ID)
     Parent parent;

     ...
}

This is quite analogous to the use of mappedBy in @OneToMany and @ManyToMany, with the difference that here mappedBy points to a different field of the current entity.

An argument against this features is that it overlaps with @MapsId, and that's true, I can use it this way:

@Entity
@IdClass(ChildId.class)
class Child {
     @Id 
     Long id;

     @Id
     Long parentId;

     @ManyToOne(mappedBy=Child.PARENT_ID)
     Parent parent;

     ...
}

or this way:

@Entity
class Child {
     @Id 
     Long id;

     @OneToOne(mappedBy=Child.ID)
     Parent parent;

     ...
}

to do things that are already possible with @MapsId.

On the other hand, I find @MapsId to be quite confusing in general, and sort-of backwards compared to my intuition. So I actually prefer the above code to the equivalent thing with @MapsId.

Reactions?

Start with the proposed @ManyToOne and @OneToOne examples in this issue, then review the linked pull request #973 and the existing @MapsId and mappedBy behavior. Done means resolving the API design and documenting or implementing an agreed way to map these associations to another field of the same entity.