使用联接表将JPA双向@ManyToOne关系映射到多个表

Question

New with JPA and ORM, so this question may be old hat. I have the following tables:

CREATE TABLE INSTITUTION (
    inst_id BIGINT PRIMARY KEY,
    :
    :
);

and

CREATE TABLE PERSON (
    pers_id BIGINT PRIMARY KEY,
    :
    :
);

each of the above tables has a one-to-many relationship to another table, CONTACTINFO

CREATE TABLE CONTACTINFO (
    cont_id BIGINT PRIMARY KEY,
    :
    :
);

In the RDBMS tables, the relationship can be mapped with two join tables:

JOIN_CONTACTS_PERSON ( cont_id, pers_id )
JOIN_CONTACTS_INSTITUTION ( cont_id, inst_id )

In SQL, the relationship can be established by joining the desired tables using the correct join table, eg:

SELECT *
  FROM Person AS p
  JOIN Join_Contacts_Person AS jt ON p.pers_id = jt.pers_id
  JOIN Contacts AS c ON jt.cont_id = c.cont_id;

In JPA, the owning side of a one-to-many relationship must be the 'many' side, in this case, the Contacts table. But there doesn't appear to be any way to map to more than one Join Table from the 'many' side using JPA annotations.

My questions are: 1) is it not possible to map a bidirectional many-to-one association through more than one join table? ... 2) Is it also not possible to map a unidirectional many-to-one relationship through two or more join tables? ... and ... 3) would a possible work-around be using the @ManyToMany annotation for two unidirectional mappings and make the "one" side the owning side of each relationship?

Answer 1

But there doesn't appear to be any way to map to more than one Join Table from the 'many' side using JPA annotations

You have a misconception here. You seem to think there is one ManyToOne association, and you would like to map it on several join tables. That's not the case. You have two different associations here, and each one has its own join table:

1. one person has many contacts, and this is mappped using the join table between contact and person
2. one institution has many contacts, and this is mapped using the join table between insitution and person

So your Contact entity would look like the following:

@Entity
public class Contact
    @Id
    private Long id;

    @ManyToOne
    @JoinTable(name = "JOIN_CONTACTS_PERSON", 
               joinColumns=
                   @JoinColumn(name="CONT_ID"),
               inverseJoinColumns=
                   @JoinColumn(name="PERS_ID"))
    private Person owningPerson;

    @ManyToOne
    @JoinTable(name = "JOIN_CONTACTS_INSTITUTION", 
               joinColumns=
                   @JoinColumn(name="CONT_ID"),
               inverseJoinColumns=
                   @JoinColumn(name="INST_ID"))
    private Institution owningInstitution;

    // ...
}