Key join columns are equivalent to standard JPA
join columns, except that they represent a join to a map key entity rather than a direct relation. You represent
a key join column with OpenJPA's
org.apache.openjpa.persistence.jdbc.KeyJoinColumn annotation. To declare a compound join, enclose an
array of KeyJoinColumns in the
org.apache.openjpa.persistence.jdbc.KeyJoinColumns
annotation.
A KeyJoinColumn always resides in
a container table, so it does not have the table property
of a standard JoinColumn. Like XJoinColumns above,
KeyJoinColumns can reference a linked field
rather than a static linked column. Otherwise, the KeyJoinColumn
and standard JoinColumn annotations are equivalent. See
Section 8.4, “
Direct Relations
” in the JPA
Overview for a review of the JoinColumn annotation.