You have a M-to-N (or Many-to-Many) relationship if an object of a class A has associated objects of class B, and class B has associated objects of class A. This relationship may be achieved through Java Set, Map, List or subclasses of these, although the only one that supports a true M-N is for a Set/Collection.

With DataNucleus this can be set up as described in this section, using what is called a Join Table relationship. Let's take the following example and describe how to model it with the different types of collection classes. We have 2 classes, Product and Supplier as below.

Here the Product class knows about the Supplier class. In addition the Supplier knows about the Product class, however with DataNucleus (as with the majority of JDO implementations) these relationships are independent.

If you define the Meta-Data for these classes as follows

<package name="mydomain">
    <class name="Product" identity-type="datastore">
        ...
        <field name="suppliers" persistence-modifier="persistent" table="PRODUCTS_SUPPLIERS">
            <collection element-type="mydomain.Supplier"/>
            <join>
                <column name="PRODUCT_ID"/>
            </join>
            <element>
                <column name="SUPPLIER_ID"/>
            </element>
        </field>
    </class>

    <class name="Supplier" identity-type="datastore">
        ...
        <field name="products" persistence-modifier="persistent" mapped-by="suppliers">
            <collection element-type="mydomain.Product"/>
        </field>
    </class>
</package>

Note how we have specified the information only once regarding join table name, and join column names as well as the <join>. This is the JDO standard way of specification, and results in a single join table.

See also :-

• M-N Worked Example
• M-N with Attributes Worked Example

Firstly a true M-N relation with Lists is impossible since there are two lists, and it is undefined as to which one applies to which side etc. What is shown below is two independent 1-N unidirectional join table relations. If you define the Meta-Data for these classes as follows

<package name="mydomain">
    <class name="Product" identity-type="datastore">
        ...
        <field name="suppliers" persistence-modifier="persistent">
            <collection element-type="mydomain.Supplier"/>
            <join/>
        </field>
    </class>

    <class name="Supplier" identity-type="datastore">
        ...
        <field name="products" persistence-modifier="persistent">
            <collection element-type="mydomain.Product"/>
            <join/>
        </field>
    </class>
</package>

There will be 4 tables, one for Product , one for Supplier , and the join tables. The difference from the Set example is in the contents of the join tables. An index column is added to keep track of the position of objects in the Lists.

In the case of a List at both ends it doesn't make sense to use a single join table because the ordering can only be defined at one side, so you have to have 2 join tables.

If you define the Meta-Data for these classes as follows

<package name="mydomain">
    <class name="Product" identity-type="datastore">
        ...
        <field name="suppliers" persistence-modifier="persistent">
            <map key-type="java.lang.String" value-type="mydomain.Supplier"/>
            <join/>
        </field>
    </class>

    <class name="Supplier" identity-type="datastore">
        ...
        <field name="products" persistence-modifier="persistent">
            <map key-type="java.lang.String" value-type="mydomain.Product"/>
            <join/>
        </field>
    </class>
</package>

This will create 4 tables in the datastore, one for Product , one for Supplier , and the join tables which also contains the keys to the Maps (a String).

Please be aware of the following.