Hibernate bag with multiple elements

I have a bag whose structure looks like this:

<bag name="foo" fetch="select" table="table_of_foos">
    <key column="foo_ids"/>
    <many-to-many class="Bar" column="bar_ids"/>
</bag>

My "table_of_foos" table has two columns: "foo_ids" and "bar_ids", neither of which is unique. I want to add another column to this table that holds some information about the relationship of foos to bars, so that my table would hold three columns. Then, I want to be able to access this information in my webapp.

Unfortunately, it doesn't seem as if I can add a property or an element to the bag. What I want to do开发者_开发知识库 is something like this:

<bag name="foo" fetch="select" table="table_of_foos">
    <key column="foo_ids"/>
    <many-to-many class="Bar" column="bar_ids"/>
    <element column="still_valid" type="integer"/>
</bag>

What is the best way to accomplish this?

---

If your joined Table needs an additional column other than its (composite) primary-key, you must implement an additional joined class.

public class Foo {

    Collection<FooBar> bar = new ArrayList<FooBar>();

}

public class Bar {

    Collection<FooBar> foo = new ArrayList<FooBar>();

}

The joined class (which needs a composite primary key - implemented as a static inner class) is described as follows

public class FooBar {

    private FooBarId fooBarId;

    private String additionalProperty;

    public static class FooBarId implements Serializable {

        private Integer fooId;
        private Integer barId;

        private Foo foo;
        private Bar bar;

        // getter's and setter's

        public FooBarId() {}
        public FooBarId(Integer fooId, Integer barId) {
            this.fooId = fooId;
            this.barId = barId;
        }

        public boolean equals(Object o) {
            if(!(o instanceof FooBarId))
                return false;

            FooBarId other = (FooBarId) o;
            return new EqualsBuilder()
                       .append(getFooId(), other.getFooId())
                       .append(getBarId(), other.getBarId())
                       .isEquals();
        }

        public int hashCode() {
            return new HashCodeBuilder()
                       .append(getFooId())
                       .append(getBarId())
                       .hashCode();
        }

    }

}

re-write your mapping as

/* Foo.class */ 

<bag name="bar" table="table_of_foos">
    <key column="BAR_ID" insert="false" update="false"/>
    <one-to-many class="FooBar" table="table_of_foos"/>
</bag>

/* Bar.class */ 

<bag name="foo" table="table_of_foos">
    <key column="FOO_ID" insert="false" update="false"/>
    <one-to-many class="FooBar" table="table_of_foos"/>
</bag>

And FooBar mapping

<class name="FooBar">
    <composite-id name="fooBarId" class="FooBar$FooBarId">
        <key-property name="fooId" column="FOO_ID"/>
        <key-property name="barId" column="BAR_ID"/>
    </composite-id>
    <property name="additionalProperty" type="string"/>
    <many-to-one name="foo" column="FOO_ID" class="Foo" insert="false" update="false"/>
    <many-to-one name="bar" column="BAR_ID" class="Bar" insert="false" update="false"/>
</class>

If you want, you can also map a composite element instead of a joined class (A component element does not need a (composite) primary key and has its has its lifecycle bound to that of its owning Entity instance. Keep this in mind)

Create you composite element (now without identifier)

public class FooBar {

    private String additionalProperty;

    private Foo foo;
    private Bar bar;

}

And define the following mapping

/* Foo.class */ 

<bag name="bar" table="table_of_foos">
    <key column="BAR_ID"/>
    <composite-element class="FooBar">
        <property name="additionalProperty" type="string"/>
        <many-to-one name="foo" column="FOO_ID" class="Foo" insert="false" update="false"/>
        <many-to-one name="bar" column="BAR_ID" class="Bar"/>
    </composite-element>
</bag>

/* Bar.class */ 

<bag name="foo" table="table_of_foos">
    <key column="FOO_ID"/>
    <composite-element class="FooBar">
        <property name="additionalProperty" type="string"/>
        <many-to-one name="foo" column="FOO_ID" class="Foo" insert="false" update="false"/>
        <many-to-one name="bar" column="BAR_ID" class="Bar"/>
    </composite-element>
</bag>

---

I wasn't able to get these mapping to work as written by Arthur Ronald F D Garcia. This code didn't work and resulted in a MappingException.

Invalid Attempt

<set name="StoreDepartments" table="`ou_store_org_unit`">
    <key column="`ou_id`" />
    <composite-element class="NHibernate.Map.OrganizationUnit+StoreDepartment+Relation, CentralDataLayer">
        <property name="Id" column="`ou_store_id`" type="Int32" />
        <many-to-one name="OrganizationUnit" column="`ou_id`" class="NHibernate.Map.OrganizationUnit, CentralDataLayer" />
        <many-to-one name="StoreDepartment" column="`store_ou_id`" class="NHibernate.Map.OrganizationUnit+StoreDepartment, CentralDataLayer" />
    </composite-element>
</set>

Resulting Error: Repeated column in mapping for collection:
NHibernate.Map.OrganizationUnit.StoreDepartments column: ou_id

I was able to get it working with the following.

Valid Attempt

<set name="StoreDepartments" table="`ou_store_org_unit`">
    <key column="`ou_id`" />
    <composite-element class="NHibernate.Map.OrganizationUnit+StoreDepartment+Relation">
        <parent name="OrganizationUnit" />
        <property name="Id" column="`ou_store_id`" type="Int32" />
        <many-to-one name="StoreDepartment" column="`store_ou_id`" class="NHibernate.Map.OrganizationUnit+StoreDepartment" />
    </composite-element>
</set>

Additionally:

If nHibernate tries to update your set even if you don't modify it, try overriding the GetHasCode and Equals methods. More information here.