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Hibernate Core Reference Guide
6.2.4. Collections of values and many-to-many associations

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

Any collection of values or many-to-many associations 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 use the <element> tag. For example:

<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"
/>

<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"
/>

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	`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"
    />

<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"
    />

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	`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): enables 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.

Here are some examples.

A set of strings:

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

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

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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>

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

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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>

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

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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>

<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>

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A list of components (this is 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>

<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>

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

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