持久性错误消息:查找操作错误地提供了一个空PK的实例

I am trying to use Netbeans 7.01 to follow a tutorial on JSF 2.0 and JPA. I am using oracle XE and JDBC_6. I used JSF pages from entities wizard to generate my JSF pages. Everything works fine as I can retrive data from the database and display them. However when I attempt to create or update a record in the database, I get this error:I am trying to use Netbeans 7.01 to follow a tu