This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
To elaborate further. Relationship tables require the primary key information of the parent table of the relationship. Some times primary key generation is deferred to the database(e.g. trigger based PK generation). In such cases PKs are generated on completion of the ejbCreate method invocation. Hence an ejbPostCreate method is provided to retrieve the PK and assign it to the relationship tables.