Jpa unidirectional relation in the parent entity

Question

I want to make unidirectional relation in the parent entity, Child entities have FK from the parent entity.

class Person {

    @Id
    @Column(name = "id")
    String id;


    @OneToOne
    @JoinColumn(name = "id", referencedColumnName = "person_id")
    Name name;
}

class Name {

    @Id
    @Column(name = "person_id")
    String personId;

    String name;
}

The Name table has FK from the person table. But I just want to define the relation in person entity only. I used @JoinColumn for mapping and set name as id from the person table

In the next example, I have one to many relations for example

class Parent {

    @Id
    @Column(name = "id")
    String id;

    @OneToMany
    @JoinColumn(name = "parent_id", referencedColumnName = "id")
    List<Child> childs;
    // fields...
}

class Child {

    @Id
    @Column(name = "Id")
    String id;


    @Column(name = "parent_id)
          String parentId;
}

In one to many relation in the @JoinColumn, I set name from the child table, and reference column from the Parent table. But when I defined the same unidirectional relation in the above person class with @OneToOne I have to set the @joincolumn name from the Person table and not from the Name table.

My question is why the defining name in these two join conditions are different, In one I have to set the name as the name of the PK column name from the same table, and in the second example, I have to set the name of the column from the child table.

Answer 1

The reason is that join column can be present only on ToOne side. As it's stated in the documentation :

public abstract java.lang.String name

(Optional) The name of the foreign key column. The table in which it is found depends upon the context.

• If the join is for a OneToOne or ManyToOne mapping using a foreign key mapping strategy, the foreign key column is in the table of the source entity or embeddable.
• If the join is for a unidirectional OneToMany mapping using a foreign key mapping strategy, the foreign key is in the table of the target entity.
• If the join is for a ManyToMany mapping or for a OneToOne or bidirectional ManyToOne/OneToMany mapping using a join table, the foreign key is in a join table.
• If the join is for an element collection, the foreign key is in a collection table.

Please note that the ability to use unidirectional @OneToMany with @JoinColumn was added only in JPA 2.0

@OneToMany
@JoinColumn(name = "parent_id")
List<Child> childs;

When you use unidirectional @OneToMany without @JoinColumn :

@OneToMany
List<Child> childs;

hibernate, for example, resorts to using a link table between the two joining entities.