JDO Guides : M-N Attributed Relation

DataNucleus provides support for standard JDO M-N relations where we have a relation between, for example, Customer and Supplier , where a Customer has many Supplier s and a Supplier has many Customer s. A slight modification on this is where you have the relation carrying some additional attributes of the relation. Let's take some classes 

public class Customer
{
    private long id; // PK
    private String name;
    private Set supplierRelations = new HashSet();

    ...
}

public class Supplier
{
    private long id; // PK
    private String name;
    private Set customerRelations = new HashSet();

    ...
}

Now we obviously cant define an "attributed relation" using Java and just these classes so we invent an intermediate "associative" class, that will also contain the attributes. 

public class BusinessRelation
{
    private Customer customer; // PK
    private Supplier supplier; // PK
    private String relationLevel;
    private String meetingLocation;

    public BusinessRelation(Customer cust, Supplier supp, String level, String meeting)
    {
        this.customer = cust;
        this.supplier = supp;
        this.relationLevel = level;
        this.meetingLocation = meeting;
    }
    ...
}

So we define the metadata like this 

<jdo>
    <package name="mydomain.business">
        <class name="Customer" detachable="true" table="CUSTOMER">
            <field name="id" primary-key="true" value-strategy="increment" column="ID"/>
            <field name="name" column="NAME"/>
            <field name="supplierRelations" persistence-modifier="persistent" mapped-by="customer">
                <collection element-type="BusinessRelation"/>
            </field>
        </class>

        <class name="Supplier" detachable="true" table="SUPPLIER">
            <field name="id" primary-key="true" value-strategy="increment" column="ID"/>
            <field name="name" column="NAME"/>
            <field name="customerRelations" persistence-modifier="persistent" mapped-by="supplier">
                <collection element-type="BusinessRelation"/>
            </field>
        </class>

        <class name="BusinessRelation" type="application" detachable="true"
             objectid-class="BusinessRelation$PK" table="BUSINESSRELATION">
            <field name="customer" primary-key="true" column="CUSTOMER_ID"/>
            <field name="supplier" primary-key="true" column="SUPPLIER_ID"/>
            <field name="relationLevel" column="RELATION_LEVEL"/>
            <field name="meetingLocation" column="MEETING_LOCATION"/>
        </class>
    </package>
</jdo>

So we've used a 1-N "CompoundIdentity" relation between Customer and BusinessRelation , and similarly between Supplier and BusinessRelation meaning that BusinessRelation has a composite PK define like this 

public class BusinessRelation
{
    ...

    public static class PK implements Serializable
    {
        public LongIdentity customer; // Use same name as BusinessRelation field
        public LongIdentity supplier; // Use same name as BusinessRelation field

        public PK()
        {
        }

        public PK(String s)
        {
            StringTokenizer st = new StringTokenizer(s, "::");
            this.customer = new LongIdentity(Customer.class, st.nextToken());
            this.supplier = new LongIdentity(Supplier.class, st.nextToken());
        }

        public String toString()
        {
            return (customer.toString() + "::" + supplier.toString());
        }

        public int hashCode()
        {
            return customer.hashCode() ^ supplier.hashCode();
        }

        public boolean equals(Object other)
        {
            if (other != null && (other instanceof PK))
            {
                PK otherPK = (PK)other;
                return this.customer.equals(otherPK.customer) && this.supplier.equals(otherPK.supplier);
            }
            return false;
        }
    }
}

This arrangement will result in the following schema

So all we need to do now is persist some objects using these classes 

PersistenceManagerFactory pmf = JDOHelper.getPersistenceManagerFactory("jpox.properties");
PersistenceManager pm = pmf.getPersistenceManager();
Transaction tx = pm.currentTransaction();
Object holderId = null;
try
{
    tx1.begin();

    Customer cust1 = new Customer("Web design Inc");
    Supplier supp1 = new Supplier("DataNucleus Corporation");
    pm.makePersistent(cust1);
    pm.makePersistent(supp1);

    BusinessRelation rel_1_1 = new BusinessRelation(cust1, supp1, "Very Friendly", "Hilton Hotel, London");
    cust1.addRelation(rel_1_1);
    supp1.addRelation(rel_1_1);
    pm.makePersistent(rel_1_1);

    tx.commit();
}
finally
{
    if (tx1.isActive())
    {
        tx1.rollback();
    }
    pm1.close();
}

This will now have persisted an entry in table "CUSTOMER", an entry in table "SUPPLIER", and an entry in table "BUSINESSRELATION". We can now utilise the BusinessRelation objects to update the attributes of the M-N relation as we wish.