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.