Any collection of values or many-to-many association requires a dedicated collection table with a foreign key column or columns, collection element column or columns and possibly an index column or columns.

For a collection of values, we use the <element> tag.

<element
        column="column_name"
        formula="any SQL expression"
        type="typename"
        length="L"
        precision="P"
        scale="S"
        not-null="true|false"
        unique="true|false"
        node="element-name"
/>

column (optional): The name of the column holding the collection element values.

formula (optional): An SQL formula used to evaluate the element.

type (required): The type of the collection element.

A many-to-many association is specified using the <many-to-many> element.

<many-to-many
        column="column_name"
        formula="any SQL expression"
        class="ClassName"
        fetch="select|join"
        unique="true|false"
        not-found="ignore|exception"
        entity-name="EntityName"
        property-ref="propertyNameFromAssociatedClass"
        node="element-name"
        embed-xml="true|false"
    />

column (optional): The name of the element foreign key column.

formula (optional): An SQL formula used to evaluate the element foreign key value.

class (required): The name of the associated class.

fetch (optional - defaults to join): enables outer-join or sequential select fetching for this association. This is a special case; for full eager fetching (in a single SELECT) of an entity and its many-to-many relationships to other entities, you would enable join fetching not only of the collection itself, but also with this attribute on the <many-to-many> nested element.

unique (optional): Enable the DDL generation of a unique constraint for the foreign-key column. This makes the association multiplicity effectively one to many.

not-found (optional - defaults to exception): Specifies how foreign keys that reference missing rows will be handled: ignore will treat a missing row as a null association.

entity-name (optional): The entity name of the associated class, as an alternative to class.

property-ref: (optional) The name of a property of the associated class that is joined to this foreign key. If not specified, the primary key of the associated class is used.

Some examples, first, a set of strings:

<set name="names" table="person_names">
    <key column="person_id"/>
    <element column="person_name" type="string"/>
</set>

A bag containing integers (with an iteration order determined by the order-by attribute):

<bag name="sizes" 
        table="item_sizes" 
        order-by="size asc">
    <key column="item_id"/>
    <element column="size" type="integer"/>
</bag>

An array of entities - in this case, a many to many association:

<array name="addresses" 
        table="PersonAddress" 
        cascade="persist">
    <key column="personId"/>
    <list-index column="sortOrder"/>
    <many-to-many column="addressId" class="Address"/>
</array>

A map from string indices to dates:

<map name="holidays" 
        table="holidays" 
        schema="dbo" 
        order-by="hol_name asc">
    <key column="id"/>
    <map-key column="hol_name" type="string"/>
    <element column="hol_date" type="date"/>
</map>

A list of components (discussed in the next chapter):

<list name="carComponents" 
        table="CarComponents">
    <key column="carId"/>
    <list-index column="sortOrder"/>
    <composite-element class="CarComponent">
        <property name="price"/>
        <property name="type"/>
        <property name="serialNumber" column="serialNum"/>
    </composite-element>
</list>