Many-to-many with additional attribute and without mapping many-to-many as separate entity

Question

the below example works fine, hovewer I would like you to check whether this is good approach, as I haven't had such requirement before and I am curious how it should be done.

I have three tables: EMPLOYEE, MEETING, EMPLOYEE_MEETING and only TWO mappings for MANY-TO-MANY relationship betweene EMPLOYEE and MEETING. It seems I am able to insert, load and update everything. What I wanted to achieve was not to create additional mapping, DAO, and separate model for the relation.

CREATE TABLE "employee_meeting" (
  "employee_id" bigint(20) NOT NULL,
  "meeting_id" bigint(20) NOT NULL,
  "opinion" varchar(50) DEFAULT NULL,
  PRIMARY KEY ("employee_id","meeting_id"),
 KEY "FK_MEETING" ("meeting_id"),
CONSTRAINT "FK_EMPLOYEE" FOREIGN KEY ("employee_id") REFERENCES "employee"      ("employee_id"),
 CONSTRAINT "FK_MEETING" FOREIGN KEY ("meeting_id") REFERENCES "meeting" ("meeting_id")
) ENGINE=InnoDB DEFAULT CHARSET=latin1$$


<hibernate-mapping package="net.viralpatel.hibernate">

<class name="Employee" table="EMPLOYEE">
    <id name="employeeId" column="EMPLOYEE_ID">
        <generator class="native" />
    </id>

    <property name="firstname" />
    <property name="lastname" column="lastname" />


<set name="meetings" table="EMPLOYEE_MEETING" inverse="false" lazy="false" fetch="select" cascade="all" >
    <key column="EMPLOYEE_ID" />
    <composite-element class="EmployeeMeeting">
        <parent name="employee" />
        <many-to-one name="meeting" column="MEETING_ID" not-null="true" cascade="all"
            class="Meeting" />
        <property name="opinion" column="OPINION" />
    </composite-element>
</set> 


</class>

<class name="Meeting" table="MEETING">

    <id name="meetingId" type="java.lang.Long"
        column="MEETING_ID">
        <generator class="native" />
    </id>

    <property name="subject" column="SUBJECT" />
    <property name="meetingDate" type="date" column="MEETING_DATE" />


</class>

 public static void main(String[] args) {

    SessionFactory sf = HibernateUtil.getSessionFactory();
    Session session = sf.openSession();
    session.beginTransaction();

    Meeting meeting1 = new Meeting("Quaterly Sales meeting");
    Meeting meeting2 = new Meeting("Weekly Status meeting");

    Employee employee1 = new Employee("Sergey", "Brin");
    Employee employee2 = new Employee("Larry", "Page");

    session.save(meeting1);

    EmployeeMeeting em = new EmployeeMeeting(employee1, meeting1, "BAD");

    employee1.getMeetings().add(em);

    session.save(employee1);
    session.save(employee2);

    session.getTransaction().commit();

    session.evict(employee1);
    session.beginTransaction();

    Employee e1 = (Employee) session.load(Employee.class, employee1.getEmployeeId());

    EmployeeMeeting em2 = e1.getMeetings().iterator().next();
    System.out.println("Test1:" + em2.getOpinion());

    em2.setOpinion("BETTER");
    session.save(e1);

    session.getTransaction().commit();

    session.evict(e1);
    session.beginTransaction();

    Employee e2 = (Employee) session.load(Employee.class, employee1.getEmployeeId());

    System.out.println("Test2:" + e2.getMeetings().iterator().next().getOpinion());
    System.out.println("Test2:" + e2.getMeetings().iterator().next().getMeeting().getMeetingId());
    System.out.println("Test2:" + e2.getMeetings().iterator().next().getEmployee().getEmployeeId());
     [/code]

Answer 1

Reduce Manty-to-Many collection in these way, without bi-directional, is equal than OneToMany collections.

I suggest to respect Many-to-Many collection, create under Meeting class the array of Employee and define a bi-directional mapping.

What's version of hibernate is used ?

Answer 2

From meeting class I dont need to acess employees.. I only need to access meetings from employees level - Having employee I need to insert, update, delete meetings.

I've made some tests, it seems to be working. One strange thing is when I run this code - I am only loading data from database:

Employee e2 = (Employee) session.load(Employee.class, new Long(18));
    Employee e = (Employee) session.load(Employee.class, new Long(17));
    Iterator i = e.getMeetings().iterator();
    while (i.hasNext()){
        System.out.println(((EmployeeMeeting) i.next()).getMeeting().getMeetingId());
    }
    System.out.println(e2.getMeetings().iterator().next().getMeeting().getMeetingId());

    session.getTransaction().commit();
    session.close();

Why HIbernate DELETES and INSERTS meetings when I am only loading data from database???

 Hibernate: select employee0_.EMPLOYEE_ID as EMPLOYEE1_0_0_, employee0_.firstname as     firstname0_0_, employee0_.lastname as lastname0_0_ from EMPLOYEE employee0_ where     employee0_.EMPLOYEE_ID=?
 Hibernate: select meetings0_.EMPLOYEE_ID as EMPLOYEE1_0_, meetings0_.MEETING_ID as      MEETING2_0_, meetings0_.OPINION as OPINION0_ from EMPLOYEE_MEETING meetings0_           where meetings0_.EMPLOYEE_ID=?
 Hibernate: select employee0_.EMPLOYEE_ID as EMPLOYEE1_0_0_, employee0_.firstname as firstname0_0_, employee0_.lastname as lastname0_0_ from EMPLOYEE employee0_ where employee0_.EMPLOYEE_ID=?
 Hibernate: select meetings0_.EMPLOYEE_ID as EMPLOYEE1_0_, meetings0_.MEETING_ID as MEETING2_0_, meetings0_.OPINION as OPINION0_ from EMPLOYEE_MEETING meetings0_ where meetings0_.EMPLOYEE_ID=?
12
11
7
13
7
Hibernate: delete from EMPLOYEE_MEETING where EMPLOYEE_ID=? and MEETING_ID=?
Hibernate: insert into EMPLOYEE_MEETING (EMPLOYEE_ID, MEETING_ID, OPINION) values (?, ?, ?)
Hibernate: delete from EMPLOYEE_MEETING where EMPLOYEE_ID=? and MEETING_ID=?
Hibernate: delete from EMPLOYEE_MEETING where EMPLOYEE_ID=? and MEETING_ID=?
Hibernate: delete from EMPLOYEE_MEETING where EMPLOYEE_ID=? and MEETING_ID=?
Hibernate: delete from EMPLOYEE_MEETING where EMPLOYEE_ID=? and MEETING_ID=?
Hibernate: insert into EMPLOYEE_MEETING (EMPLOYEE_ID, MEETING_ID, OPINION) values (?,   ?, ?)
Hibernate: insert into EMPLOYEE_MEETING (EMPLOYEE_ID, MEETING_ID, OPINION) values (?, ?, ?)
Hibernate: insert into EMPLOYEE_MEETING (EMPLOYEE_ID, MEETING_ID, OPINION) values (?, ?, ?)
Hibernate: insert into EMPLOYEE_MEETING (EMPLOYEE_ID, MEETING_ID, OPINION) values (?, ?, ?)