6 Entity Relationship Mapping

This chapter discusses how to develop entity-to-entity relationships. As a developer, you can approach entity relationships from either of the following viewpoint:

• EJB development—You can use UML diagrams to design the entity beans, and the cardinality and direction of the relationship between each bean, from the perspective of the EJB objects.

• Database development—You can use ERD diagrams to design the database tables, complete with the cardinality and direction designated by primary and foreign keys, that support the entity beans. The focus is on how the database maps each entity bean and the relationships between them.

This chapter starts by discussing entity relationships from the EJB development viewpoint. Next, it demonstrates how the deployment descriptor maps to database tables. If you want to design with the database development viewpoint, skip to "Mapping Object Relationship Fields to the Database".

Note: An object-relationship entity bean example (ormapdemo.jar) is available on OTN from the OC4J sample code page at http://www.oracle.com/technology/tech/java/oc4j/demos/ on the OTN Web site.

This chapter covers the following topics:

• Transaction Requirements

• Defining Entity-To-Entity Relationships

• Mapping Object Relationship Fields to the Database

• Using a Foreign Key in a Composite Primary Key

• How to Override a Foreign Key Database Constraint