8.2. Collections of dependent objects

Collections of components are supported (eg. an array of type Name). Declare your component collection by replacing the <element> tag with a <composite-element> tag.
<set name="someNames" table="some_names" lazy="true">
    <key column="id"/>
    <composite-element class="eg.Name"> <!-- class attribute required -->
        <property name="initial"/>
        <property name="first"/>
        <property name="last"/>
    </composite-element>
</set>
Note: if you define a Set of composite elements, it is very important to implement equals() and hashCode() correctly.
Composite elements may contain components but not collections. If your composite element itself contains components, use the <nested-composite-element> tag. This is a pretty exotic case - a collection of components which themselves have components. By this stage you should be asking yourself if a one-to-many association is more appropriate. Try remodelling the composite element as an entity - but note that even though the Java model is the same, the relational model and persistence semantics are still slightly different.
Please note that a composite element mapping doesn't support null-able properties if you're using a <set>. Hibernate has to use each columns value to identify a record when deleting objects (there is no separate primary key column in the composite element table), which is not possible with null values. You have to either use only not-null properties in a composite-element or choose a <list>, <map>, <bag> or <idbag>.
A special case of a composite element is a composite element with a nested <many-to-one> element. A mapping like this allows you to map extra columns of a many-to-many association table to the composite element class. The following is a many-to-many association from Order to Item where purchaseDate, price and quantity are properties of the association:
<class name="eg.Order" .... >
    ....
    <set name="purchasedItems" table="purchase_items" lazy="true">
        <key column="order_id">
        <composite-element class="eg.Purchase">
            <property name="purchaseDate"/>
            <property name="price"/>
            <property name="quantity"/>
            <many-to-one name="item" class="eg.Item"/> <!-- class attribute is optional -->
        </composite-element>
    </set>
</class>
Of course, there can't be a reference to the purchae on the other side, for bidirectional association navigation. Remember that components are value types and don't allow shared references. A single Purchase can be in the set of an Order, but it can't be referenced by the Item at the same time.
Even ternary (or quaternary, etc) associations are possible:
<class name="eg.Order" .... >
    ....
    <set name="purchasedItems" table="purchase_items" lazy="true">
        <key column="order_id">
        <composite-element class="eg.OrderLine">
            <many-to-one name="purchaseDetails class="eg.Purchase"/>
            <many-to-one name="item" class="eg.Item"/>
        </composite-element>
    </set>
</class>
Composite elements may appear in queries using the same syntax as associations to other entities.