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Oracle TopLink Developer's Guide
10g Release 3 (10.1.3) B13593-01 |
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Oracle TopLink Developer's Guide
10g Release 3 (10.1.3) B13593-01 |
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Aggregate collection mappings are used to represent the aggregate relationship between a single-source object and a collection of target objects. Unlike the TopLink one-to-many mappings, in which there should be a one-to-one back reference mapping from the target objects to the source object, there is no back reference required for the aggregate collection mappings, because the foreign key relationship is resolved by the aggregation.
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Note: To use aggregate collections with TopLink Workbench, you must use an amendment method (see "Configuring Amendment Methods"), or manually edit the project source to add the mapping. |
Although aggregate collection mappings are similar to one-to-many mappings, they are not replacements for one-to-many mappings. Use aggregate collections only in situations where the target collections are of a reasonable size and if having a one-to-one back mapping is difficult.
Because one-to-many relationships offer better performance and are more robust and scalable, consider using a one-to-many relationship rather than an aggregate collection. In addition, aggregate collections are privately owned by the source of the relationship and must not be shared or referenced by other objects.
Aggregate collections are privately owned by the source of the relationship and should not be shared or referenced by other objects.
This section describes the following:
See "Configuring a Relational Aggregate Collection Mapping" for more information.
Aggregate collection descriptors can use inheritance. You must also declare subclasses as aggregate collection. The subclasses can have their own mapped tables, or share the table with their parent class. See "Descriptors and Inheritance" for more information on inheritance.
In a Java Vector, the owner references its parts. In a relational database, the parts reference their owners. Relational databases use this implementation to make querying more efficient.
Aggregate collection mappings require a target table for the target objects.
To implement an aggregate collection mapping, the following must take place:
The descriptor of the target class must declare itself as an aggregate collection object. Unlike the aggregate object mapping, in which the target descriptor does not have a specific table to associate with, there must be a target table for the target object.
The descriptor of the source class must add an aggregate collection mapping that specifies the target class.
You can use aggregate collection mappings with entity beans if the source of the relationship is an entity bean or Java object, and the mapping targets are regular Java objects. Entity beans cannot be the target of an aggregate object mapping.
To implement an aggregate collection mapping, the following must take place:
The descriptor of the target class must declare itself to be an aggregate collection object. Unlike the aggregate object mapping, in which the target descriptor does not have a specific table to associate with, there must be a target table for the target object.
The descriptor of the source class must add an aggregate collection mapping that specifies the target class.
