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 Collection, Set, List or subclasses of these, although the only one that supports a true M-N is Set.

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 these relationships are really independent.

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

<entity-mappings>
    <entity class="mydomain.Product">
        <table name="PRODUCT"/>
        <attributes>
            <id name="id">
                <column name="PRODUCT_ID"/>
            </id>
            <basic name="name">
                <column name="NAME"/>
            </basic>
            <basic name="price">
                <column name="PRICE"/>
            </basic>
            <many-to-many name="suppliers" mapped-by="products">
                <join-table name="PRODUCTS_SUPPLIERS">
                    <join-column name="PRODUCT_ID"/>
                    <inverse-join-column name="SUPPLIER_ID"/>
                </join-table>
            </many-to-many>
        </attributes>
    </entity>

    <entity class="mydomain.Supplier">
        <table name="SUPPLIER"/>
        <attributes>
            <id name="id">
                <column name="SUPPLIER_ID"/>
            </id>
            <basic name="name">
                <column name="NAME"/>
            </basic>
            <many-to-many name="products"/>
        </attributes>
    </entity>
</entity-mappings>

Note how we have specified the information only once regarding join table name, and join column names as well as the <join-table>. This is the JPA standard way of specification, and results in a single join table. The "mapped-by" ties the two fields together.

See also :-

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

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

<entity-mappings>
    <entity class="mydomain.Product">
        <table name="PRODUCT"/>
        <attributes>
            <id name="id">
                <column name="PRODUCT_ID"/>
            </id>
            <basic name="name">
                <column name="NAME"/>
            </basic>
            <basic name="price">
                <column name="PRICE"/>
            </basic>
            <many-to-many name="suppliers" mapped-by="products">
                <order-by>name</order-by>
                <join-table name="PRODUCTS_SUPPLIERS">
                    <join-column name="PRODUCT_ID"/>
                    <inverse-join-column name="SUPPLIER_ID"/>
                </join-table>
            </many-to-many>
        </attributes>
    </entity>

    <entity class="mydomain.Supplier">
        <table name="SUPPLIER"/>
        <attributes>
            <id name="id">
                <column name="SUPPLIER_ID"/>
            </id>
            <basic name="name">
                <column name="NAME"/>
            </basic>
            <many-to-many name="products">
                <order-by>name</order-by>
            </many-to-many>
        </attributes>
    </entity>
</entity-mappings>

There will be 3 tables, one for Product , one for Supplier , and the join table. The difference from the Set example is that we now have <order-by> at both sides of the relation. This has no effect in the datastore schema but when the Lists are retrieved they are ordered using the specified order-by.

Please be aware of the following.