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8. Entity Map


Definition:
An entity map is a map who is keyed by an entity class (i.e. First-rank) rather than a simple property.

Scenario:
Foo holds a Map of people's ages. This map is keyed by the Name entity class. Name holds a person's name as a string property.

Map Foo.getAges()     //  returns a collection of Person-String instances

Hibernate Mapping:
In Hibernate, this could be mapped as follows:

<class name="Foo" table="foo">
     ...
     <map role="ages">
         <key column="id"/>
         <index-many-to-many column="person_id" class="Person"/>
         <element column="age" type="string"/>
     </map>
 </class>
<class name="Person" table="person">
     ...
     <property name="name" column="name" type="string"/>
 </class>

Table Schema:

Foo

id

Person

id

name

Ages

id

person_id

age


As for the normal map a simple extra table, Ages, is used to store the Person FK and age data.

Bidirectionality:
Bidirectionality has no meaning for an entity map.

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