entity manager merge() issue with id null - jpa

Question

I'm encountering a problem when merging an existing parent entity with a new child entity.
Here's the scenario:
Create two new parent's and persist in db.
In parent's table we will have two new entries as follows
ID = 0, NAME = A
ID = 1, NAME = B
Using entitymanager.find() , fetch the parent with ID=0, and create a new child for the parent.
In the child's table, we will have the following entry
ID=0, NAME = CHILD of A, PARENT_ID = 0
Using entitymanager.find() , fetch the parent with ID=1, and create a new child for the parent.
After merging the parent object using entitymanager.merge(), the newly created child get's merge with the existing child with ID = 0.
The entries in child table, becomes ID=0 , CHILD OF PARENT B, PARENT_ID =0.
Why does this happen? Shouldn't it create a new entry for Child ?
The parent entity:
public class ParentEntity implements Serializable {
/**
*
*/
private static final long serialVersionUID = 1L;
#Id
#GeneratedValue(strategy=GenerationType.SEQUENCE,generator="PARENT_SEQ")
#SequenceGenerator(sequenceName="PARENT_SEQ",schema="MURTAZA YAHYA", name = "PARENT_SEQ", initialValue=-1)
//#GeneratedValue(strategy=GenerationType.IDENTITY)
#Column(name="ID",unique=true, nullable=false)
private Integer id;
#Column(name="NAME" , length=150)
private String name;
#OneToMany(mappedBy="parentEntity",cascade={CascadeType.MERGE,CascadeType.PERSIST})
List<ChildEntity> childEntities;
public Integer getId() {
if(id == null)
return null;
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public List<ChildEntity> getChildEntities() {
return childEntities;
}
public void setChildEntities(List<ChildEntity> childEntities) {
this.childEntities = childEntities;
}
}
The Child Entity
public class ChildEntity implements Serializable{
private static final long serialVersionUID = 1L;
#Id
#GeneratedValue(strategy=GenerationType.SEQUENCE,generator="CHILD_SEQ")
#SequenceGenerator(sequenceName="CHILD_SEQ",schema="MURTAZA YAHYA", name = "CHILD_SEQ" , initialValue=0)
//#GeneratedValue(strategy=GenerationType.IDENTITY)
#Column(name="ID",unique=true, nullable=false)
private Integer id;
#Column(name="NAME" , length=150)
private String name;
#ManyToOne
#JoinColumn(name="PARENT_ID")
private ParentEntity parentEntity;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public ParentEntity getParentEntity() {
return parentEntity;
}
public void setParentEntity(ParentEntity parentEntity) {
this.parentEntity = parentEntity;
}
}
public String AddChildToAnExistingParent(Parent p) {
// TODO Auto-generated method stub
ParentEntity parent = em.find(ParentEntity.class, p.getId());
ChildEntity childEntity = new ChildEntity();
childEntity.setName("INISYA");
childEntity.setParentEntity(parent);
List<ChildEntity> list = new ArrayList<ChildEntity>();
list.add(childEntity);
parent.setChildEntities(list);
em.merge(parent);
return "Success";
}
Any help will be greatly appreciated. Thank you,

Answer 1

As the Parent->Child association is annotated with CascadeType.PERSIST the following code should resolved the described scenario.
public String AddChildToAnExistingParent(Parent p)
{ // TODO Auto-generated method stub
ParentEntity parent = em.find(ParentEntity.class, p.getId());
ChildEntity childEntity = new ChildEntity();
childEntity.setName("INISYA");
childEntity.setParentEntity(parent);
parent.getChildEntities().add (childEntity);
em.persist(parent);// the cascade.persist cause new childs persistence/insert
return "Success";
}
For more details in this answer explains the behavior of EntityManager method merge and persist.