6.2.4. Collections of values and many-to-many associations
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>
