Resin Documentation

collections: the @onetomany relation

The @OneToMany relation adds collection extensions to the query language and provides a Java Collection containing the children. @OneToMany represents a collection of children belonging to a parent, like students in Gryffindor house at Hogwarts school.

The Many-To-One tutorial illustrated that a many-to-one relation links one source entity to another target entity. A one-to-many relation links the target entity back to the source entity.

In this example, each House has many Students, each Student has one House. House has a one-to-many relationship with Student, Student has a many-to-one relationship with House

Demo

CREATE TABLE house (
  id BIGINT PRIMARY KEY auto_increment,

  name VARCHAR(250),
)

CREATE TABLE student (
  id BIGINT PRIMARY KEY auto_increment,

  name VARCHAR(250),

  house BIGINT REFERENCES house(id)
)

  @OneToMany(mappedBy="house")
  private Set<Student> _students;

  @ManyToOne
  @JoinColumn(name="house")
  private House _house;

The one-to-many relation provides a House the ability to get all of its Students. Resin will perform any necessary database lookup.

The example queries all houses and prints their names and all of their students.

private void doService(PrintWriter out)
  throws java.io.IOException
{
  public void service(HttpServletRequest req, HttpServletResponse res)
    throws java.io.IOException, ServletException
  {
    PrintWriter out = res.getWriter();

    res.setContentType("text/html");

    String sql = "SELECT h FROM House h";
    
    Query allHouse = _entityManager.createQuery("SELECT o FROM House o");

    List houses = allHouse.getResultList();

    for (int i = 0; i < houses.size(); i++) {
      House house = (House) houses.get(i);

      out.println("<h3>" + house.getName() + "</h3>");

      for ( Student student : house.getStudents() ) {
        out.println( student.getName() + "<br>" );
      }
    }
  }
}

SELECT h.students FROM House h WHERE h.name='Gryffindor'

SELECT s FROM House h, IN(h.students) s WHERE h.name='Gryffindor'

SELECT s FROM Student s, House h WHERE s MEMBER OF h.students

SELECT h FROM House h WHERE h.students IS EMPTY

Demo