In the spec:
    2.4.1 Primary Keys Corresponding to Derived Identities
        The identity of an entity may be derived from the identity of another entity (the "parent" entity) when
        the former entity (the "dependent" entity) is the owner of a many-to-one or one-to-one relationship to
        the parent entity and a foreign key maps the relationship from dependent to parent.

For example, in the attached ERD, consider the aspectDisplay table. "Enitty Classes from Database" generates something like:

    @Entity
    @Table(name = "ASPECTDISPLAY", catalog = "", schema = "ERNIE")
    public class Aspectdisplay implements Serializable {
        private static final long serialVersionUID = 1L;
        @EmbeddedId
        protected AspectdisplayPK aspectdisplayPK;
        @Basic(optional = false)
        @Lob
        @Column(name = "DAT", nullable = false)
        private Serializable dat;
        @JoinColumns({@JoinColumn(name = "CHART_ID", referencedColumnName =
                "CHART_ID", nullable = false, insertable = false, updatable = false), @JoinColumn(name =
                "EVENT_ID", referencedColumnName = "EVENT_ID", nullable = false, insertable =
                false, updatable = false)})
        @OneToOne(optional = false)
        private ChartEvent chartEvent;

wheras instead of using @JoinColumns, it should be done with @MappedById.

    @Entity
    @Table(name = "ASPECTDISPLAY", catalog = "", schema = "ERNIE")
    public class Aspectdisplay implements Serializable {
        private static final long serialVersionUID = 1L;
        @EmbeddedId
        protected ChartEventPK chartEventPK;
        @Basic(optional = false)
        @Lob
        @Column(name = "DAT", nullable = false)
        private Serializable dat;
        @MappedById // <<<<<<<<<<<<<<<<<<<<<<< instead of @JoinColumns
        @OneToOne(optional = false)
        private ChartEvent chartEvent;

Mapping this way allows the persistence provider to automatically set the primary key (chartEvent must be set).

(Note in the JPA 1.0 release this should be mapped with @PrimaryKeyJoinColumns, @MappedById is now the preferred mechanism)

This is loosely related to Issue 172075, but there is no dependency.