Defining EJB 3 Inheritance Mappings

EJB 3 persistence supports all three popular inheritance mapping strategies and also mixed strategies.

All classes in the class hierarchy should be either Entity classes or Mapped superclasses. For each class hierarchy, the primary identifier must be defined on the Entity class that is the root of the hierarchy or on a mapped superclass of the hierarchy.

You can optionally define a Version attribute on the entity that is the root of the entity hierarchy or on a Mapped superclass of the entity hierarchy.

• Mapped Superclasses
  In EJB 3.0, Mapped superclasses are used to define state and mapping information that is common to multiple entity classes. They are not mapped to separate tables of their own. You cannot currently define Mapped superclasses in PowerDesigner.
• Table Per Class Hierarchy Strategy
  In this strategy, the whole class hierarchy is mapped to one table. You can optionally define discriminator values for each Entity class in the hierarchy on the EJB 3 Persistence tab of the class property sheet.
• Joined Subclass Strategy
  In this strategy, each class is mapped to its own primary table. Primary tables of child classes have reference keys referring to the primary tables of the parent classes.
• Applying Table Per Class Strategy
  In this strategy, each class is mapped to a separate table. When transforming an OOM to a PDM, PowerDesigner only generates tables for leaf classes, and assumes that all other classes are not mapped to a table, even if you manually define additional mappings. The MappedSuperclass annotations are generated for those classes, and the Inheritance annotation will not be generated for all the classes. You need to customize the generated annotations and create additional tables if you want to map classes other than leaf classes to tables.