Question

I have the following two entities:

class Role {
  @Id Long id;
  @Column String email;
  String role;
  @ManyToOne User user;
}

class User {
  @Id Long id;
  @Column(unique=true) String email;
  String password;

  @OneToMany(mappedBy = "user")
  @OrderBy("role asc")
  List<Role> roles;
}

The SQL is:

CREATE TABLE USER (
ID BIGINT NOT NULL, 
EMAIL VARCHAR(255) UNIQUE, 
PASSWORD VARCHAR(255), 
PRIMARY KEY (ID)
)
CREATE TABLE ROLE (
ID BIGINT NOT NULL, 
EMAIL VARCHAR(255),
ROLE VARCHAR(255), 
USER_ID BIGINT, 
PRIMARY KEY (ID),
FOREIGN KEY (USER_ID) REFERENCES USER(ID)
)

The following JUnit fails on the second last line. I get the user (the parent) automatically loaded from the child, but I don’t get the children automatically loaded from the parent. What have I done wrong?

    Role r = em.createQuery("select r from Role r where r.email = :email", Role.class)
            .setParameter("email", "[e-mail]")
            .getSingleResult();
    assertEquals("registered", r.role());
    assertEquals("[e-mail]", r.user().email());

    User u = em.createQuery("select u from User u where u.email = :email", User.class)
            .setParameter("email", "[e-mail]")
            .getSingleResult();

    assertEquals("[e-mail]", u.email());
    assertEquals("asdf", u.password());

    assertEquals(1, u.roles().size()); FAILS HERE!!!
    assertEquals("registered", u.roles().get(0).role());

I’m using EclipseLink 2.3.2 (JPA)

Answer 1

There’s multiple ways in which you can tell how the other and of a relation should be loaded. One is at the query level, using fetch join, something like this:

select u from User u left join FETCH u.roles r where...

The other is at the relation declaration level, something like this:

class User {
  @Id Long id;
  @Column(unique=true) String email;
  String password;

  @OneToMany(mappedBy = "user", fetch=FetchType.EAGER)
  @OrderBy("role asc")
  List<Role> roles;
}

The reason why when you load a Role you also get the user without any special annotation or join in your query is that ManyToOne relations in JPA have fetch=EAGER by default:

http://docs.oracle.com/javaee/5/api/javax/persistence/ManyToOne.html#fetch()

while one to many have fetch=lazy by default:

http://docs.oracle.com/javaee/5/api/javax/persistence/OneToMany.html#fetch()