1.2.5. Bi-directional associations

Next you will map a bi-directional association. You will make the association between person and event work from both sides in Java. The database schema does not change, so you will still have many-to-many multiplicity.

Note

A relational database is more flexible than a network programming language, in that it does not need a navigation direction; data can be viewed and retrieved in any possible way.

First, add a collection of participants to the Event class:

    private Set participants = new HashSet();

    public Set getParticipants() {
        return participants;
    }

    public void setParticipants(Set participants) {
        this.participants = participants;
    }

    private Set participants = new HashSet();

    public Set getParticipants() {
        return participants;
    }

    public void setParticipants(Set participants) {
        this.participants = participants;
    }

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Now map this side of the association in Event.hbm.xml.

<set name="participants" table="PERSON_EVENT" inverse="true">
  <key column="EVENT_ID"/>
  <many-to-many column="PERSON_ID" class="org.hibernate.tutorial.domain.Person"/>
</set>

<set name="participants" table="PERSON_EVENT" inverse="true">
  <key column="EVENT_ID"/>
  <many-to-many column="PERSON_ID" class="org.hibernate.tutorial.domain.Person"/>
</set>

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These are normal set mappings in both mapping documents. Notice that the column names in key and many-to-many swap in both mapping documents. The most important addition here is the inverse="true" attribute in the set element of the Event's collection mapping.

What this means is that Hibernate should take the other side, the Person class, when it needs to find out information about the link between the two. This will be a lot easier to understand once you see how the bi-directional link between our two entities is created.

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1.2.5. Bi-directional associations

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