How to map many-to-many List in Hibernate with a Link Table

Question

I would like to map a many-to-many in Hibernate using a link table. I have two classes, Parent and Child class, for example:

public class Parent{

private List<Child> _children;

//...getters and setters
}

I use a link table (link_table) with three columns link_id, parent_id, and child_id. The database is SQL server and id types are uniqueidentifier. So, I usually use guid for the id fields.

How can you implement this using the <list /> tag if this is the correct tag to use? Do you know of any good documentation to accomplish this?

I am currently getting a ConstraintViolationException but have not been able to find any good documentation or examples of this.

I think a main issue is: how to specify the link_id to be automatically generated in the link table.

Answer 1

I am not sure that you can pull this off easily for an existing database with existing data. Hibernate is usually better off defining its own data schema the first time you connect...

I've only pulled off many-to-many with annotations, but I think the hibernate documentation offers XML based examples: link text

Answer 2

I do this using annotations, specifically @ManyToMany and @JoinTable:

Hibernate Docs:

@Entity
public class Employer implements Serializable {
    @ManyToMany(
        targetEntity=org.hibernate.test.metadata.manytomany.Employee.class,
        cascade={CascadeType.PERSIST, CascadeType.MERGE}
    )
    @JoinTable(
        name="EMPLOYER_EMPLOYEE",
        joinColumns=@JoinColumn(name="EMPER_ID"),
        inverseJoinColumns=@JoinColumn(name="EMPEE_ID")
    )
    public Collection getEmployees() {
        return employees;
    }
}


@Entity
public class Employee implements Serializable {
    @ManyToMany(
        cascade = {CascadeType.PERSIST, CascadeType.MERGE},
        mappedBy = "employees",
        targetEntity = Employer.class
    )
    public Collection getEmployers() {
        return employers;
    }
}

Answer 3

I don't think that it is possible (or necessary) to add a link_id primary key to the join table. The join table will usually consist of the primary keys of the two participating tables.

Using XML you will need syntax like this:

 <class name="Parent">
    ....
    <list name="children" table="link_table">
    <key column="parent_id"/>
    <many-to-many column="child_id"
        class="Children"/>
    </list>
    ...
 </class>

<class name="Child">
...
<list name="parents" inverse="true" table="link_table">
    <key column="child_id"/>
    <many-to-many column="parent_id"
        class="Parent"/>
</list>
...
</class>

Although I find annotations better to use.