The Java EE 6 Tutorial, Volume I

Entity Mapped to More Than One Database Table

Part’s fields map to more than one database table: PERSISTENCE_ORDER_PART and PERSISTENCE_ORDER_PART_DETAIL. The PERSISTENCE_ORDER_PART_DETAIL table holds the specification and schematics for the part. The @SecondaryTable annotation is used to specify the secondary table.

@SecondaryTable(name="PERSISTENCE_ORDER_PART_DETAIL", pkJoinColumns={
public class Part {

PERSISTENCE_ORDER_PART_DETAIL shares the same primary key values as PERSISTENCE_ORDER_PART. The pkJoinColumns element of @SecondaryTable is used to specify that PERSISTENCE_ORDER_PART_DETAIL’s primary key columns are foreign keys to PERSISTENCE_ORDER_PART. The @PrimaryKeyJoinColumn annotation sets the primary key column names and specifies which column in the primary table the column refers to. In this case, the primary key column names for both PERSISTENCE_ORDER_PART_DETAIL and PERSISTENCE_ORDER_PART are the same: PARTNUMBER and REVISION, respectively.