How to specify a constant value for a JoinColumn?

naesc · **Joined:** Mon Apr 14, 2008 1:13 pm **Posts:** 11

This seems like a pretty common thing to want to do but I haven't seen any example of how to accomplish this:

Basically, I want to have a JoinColumns annotation where I specify a constant value for one of the columns.

Here's a simple example to illustrate... basically I want to bind a Person object's homeAddress value to the Address table where Address.person_id = Person.person_id AND Address.addr_type = 1.

It seems like maybe JPA doesn't solve this itself but I'm hoping Hibernate Annotations provides a solution???

Code:

@Entity
public class Person {
    @Id
    @Column(name="person_id")
    private Long id;

    @JoinColumns({
        @Column(name="person_id"),
        @Column(??? - join to address record of type 1 ???)
    })
    private Address homeAddress;

    @JoinColumns({
        @Column(name="person_id"),
        @Column(??? - join to address record of type 2 ???)
    })
    private Address longAddress;

    ...
}

@Entity
@IdClass(Address.Key.class)
public class Address {
    @Id
    private Person resident;

    @Id
    private int addressType;

    ...

    public static class Key {
        @OneToOne
        @JoinColumn(name="person_id")
        private Person resident;
        
        @Id
        @Column(name="addr_type")
        private int addressType;

        ...
    }
}

sanne.grinovero · **Posted:** Mon May 19, 2008 5:36 pm

Hi,
yes that about a constant value is often a requirement;
I usually have an enum type for the Address
(homeAddress, workAddress, ....) or you may prefer an Integer, so you can later add more types to your DB without changing the code.

The solution is to use a Map binding:

Code:

@Entity
public class Person {
    @Id
    private Long id;

    @OneToMany(mappedBy="ofPerson")
    @MapKey(name="addressType")
    private Map<AddressType,Address> addresses;
   ...
}
@Entity @IdClass(AddressPk.class)
public class Address {
    
    @Id @Enumerated
    @Column(name = "address_type_id", nullable = false, updatable = false )
    private AddressType addressType;

    @Id @ManyToOne
    @JoinColumn(name = "person_id", nullable = false, updatable = false )
    private Person ofPerson
   ...
}

You should add an appropriate AddressPk and use the same column names; if you get it right you get a very nice double key in Address: PK (type,person_id FK)