JDO Composite Key join

Question

First of all I have read the documentation (https://www.datanucleus.org/products/accessplatform_6_0/jdo/mapping.html#compound_identity_1_1_uni), but I did not understand it

The problem

I have class A that has keyA, keyB, keyC as normal fields, but class B has these as primary keys which form a composite key. There are no foreign keys in this relationship. How do I fetch B class by using the fields in class A?

I think JPA has @JoinColumns and @JoinFormula annotations or most likely @PrimaryKeyJoinColumn, but I haven't found anything similar in JDO (DataNucleus)

public class A {

    @Persistent
    @Column(jdbcType = "VARCHAR", length = 30)
    private String keyA;

    @Persistent
    @Column(jdbcType = "VARCHAR", length = 32)
    private String keyB;

    @Persistent
    @Column(jdbcType = "VARCHAR", length = 30)
    private String keyC;

    @Persistent
    private B b
}

@PersistenceCapable(detachable = "true")
public class B  {

    @PrimaryKey
    @Persistent
    @Column(jdbcType = "VARCHAR", length = 30)
    private String keyA;

    @PrimaryKey
    @Persistent
    @Column(jdbcType = "VARCHAR", length = 32)
    private String keyB;

    @PrimaryKey
    @Persistent
    @Column(jdbcType = "VARCHAR", length = 30)
    private String keyC;

    @Persistent
    @Column(jdbcType = "VARCHAR", length = 30)
    private String someDataINeed;

I checked the documentation and searched the web but couldn't find anything similar to what I want