JPA : M-N Relationships

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.

Using Set

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>
            ...
            <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>
            ...
            <many-to-many name="products"/>
        </attributes>
    </entity>
</entity-mappings>

or alternatively using annotations 

public class Product
{
    ...

    @ManyToMany(mappedBy="products")
    @JoinTable(name="PRODUCTS_SUPPLIERS")
    @JoinColumn(name="PRODUCT_ID")
    @InverseJoinColumn(name="SUPPLIER_ID")
    Collection<Supplier> suppliers
}

public class Supplier
{
    ...

    @ManyToMany
    Collection<Product> products;

    ...
}

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 :-


Using List

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>
            ...
            <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>
            ...
            <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.



Relationship Behaviour

Please be aware of the following.

  • To add an object to an M-N relationship you need to set it at both ends of the relation since the relation is bidirectional and without such information the JPA implementation won't know which end of the relation is correct.
  • If you want to delete an object from one end of a M-N relationship you will have to remove it first from the other objects relationship. If you don't you will get an error message that the object to be deleted has links to other objects and so cannot be deleted.