@Entity
public class Parent{
  @Id
  public long pid;

  public String name;

  @OneToMany(mappedBy = "parent", cascade = CascadeType.ALL)
  public List<Child> children;
}

-------------------------------------------------------------------

@Entity
public class Child{
  @EmbeddedId
  public PK childPK = new PK();

  public String name;

  @ManyToOne
  @MapsId("parentPk")
  @JoinColumn(name = "foreignKeyFromParent")
  public Parent parent;

  @Embeddable
  @EqualsAndHashCode
  static class PK implements Serializable {
      public long parentPk;
      public long cid;
  }
}
------------------------------------------------------------------------

public interface ParentRepository extends JpaRepository<AmazonTest, Long> {
}

public static void main(String[] args) {
    @Autowired
    private ParentRepository parentRepository;

    Parent parent = new Parent();
    parent.pid = 1;
    parent.name = "Parent 1";

    Child child = new Child();
    List<Child> childList = new ArrayList<>();

    child.childPK.cid = 1;
    child.name = "Child 1";
    childList.add(child);

    parent.children= childList;

    parentRepository.save(parent);
    parentRepository.flush();
}

@Entity
public class Parent{
   @Id
   public long pid;

   public String name;

   @OneToMany(mappedBy = "parent", cascade = CascadeType.ALL)
   public List<Child> children;
}

-------------------------------------------------------------------

@Entity
public class Child{
   @Id
   public long cid;


   public String name;

   @ManyToOne
   @JoinColumn(name = "foreignKeyFromParent")
   public Parent parent;

}
------------------------------------------------------------------------

public interface ParentRepository extends JpaRepository<AmazonTest, Long> {
}

-------------------------------------------------------------------------
public static void main(String[] args) {
   @Autowired
   private ParentRepository parentRepository;

   Parent parent = new Parent();
   parent.pid = 1;
   parent.name = "Parent 1";

   Child child = new Child();
   List<Child> childList = new ArrayList<>();

   child.cid = 1;
   child.name = "Child 1";
   childList.add(child);

   parent.children= childList;

   parentRepository.save(parent);
   parentRepository.flush();
}