public class MainClass {
    @Id
    @Column(name="MAIN_ID")
    private Long id;

    @OneToOne(optional=false)
    @PrimaryKeyJoinColumn   
    private PartOne part;

}

public class PartOne{
    @Id
    @Column(name="MAIN_ID")
    private Long mainId;

   //other properties   
}

public class PartOneDetails {
     private Long partOneId;
}

public class MainClass {
    @Id
    @Column(name="MAIN_ID")
    private Long id;

    @OneToOne(optional=false)
    @PrimaryKeyJoinColumn   
    private PartOne part;

}

public class PartOne{
    @Id
    @Column(name="MAIN_ID")
    private Long mainId;

    @OneToMany(cascade=CascadeType.ALL,mappedBy="mainId")
    public List<PartOneDetails> getPartDetails() {
      return partDetails;
   }
   //other properties   
}

public class PartOneDetails {
     @Id
     private Long partOneId;
     
     @ManyToOne(targetEntity=PartOne.class)
     @JoinColumn(name="MAIN_ID", nullable=false)
     @ForeignKey(name="FK_PART_ONE_DETAILS_PART_ONE")
     public Long getMainId() {
   return mainId;
     }     
}