9. 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.
Hibernate Mapping:
In Hibernate, this could be mapped as follows:
Table Schema:
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.
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:
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<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.