Cannot persist detached object with @ManyToOne

Question

I have a simple table that holds named relationships between 2 objects. I am using a @ManyToOne type from the relationship object to each of the parent and child objects.
If the parent and child have NOT already been persisted everything works fine. However if either of them have been previously persisted I fail with a "Cannot persist detached object" error. Why is this, and what is the resolution?
Through much trial and error I was never able to get em.merge to resolve the issue. However I did implement a @PrePersist and registered both parent and child in the UnitOfWork and this did make the problem go away. This 'hack' however cannot be the right solution since its not portable in any fashion across other ORM providers.
It's seems as though the Cascade.PERSIST should handle the fact that the objects have already been persisted?
The following is the relationship class:
@Entity
@Table(name="UDM_RELATIONS")
public class UdmRelationship extends UdmPersistentObject
* If the parent has not be saved then let the persist of this object take care of it.
* Don't touch the parent if this relationship is removed.
@ManyToOne(cascade=CascadeType.PERSIST)
@JoinColumn(name="PARENT_ID")
protected UdmPersistentObject parent = null;
* If the child has not be saved then let the persist of this object take care of it.
* Don't touch the parent if this relationship is removed.
@ManyToOne(cascade=CascadeType.PERSIST)
@JoinColumn(name="CHILD_ID")
protected UdmPersistentObject child = null;
* Name the relationship for easier querying and viewing of direct table data.
@Column(name="REL_NAME")
protected String relationshipName = null;
// Initialization
public UdmRelationship(String relationshipName)
this.relationshipName = relationshipName;
public UdmRelationship()
Annotation annotation = this.getClass().getAnnotation(DiscriminatorValue.class);
if ( annotation != null )
DiscriminatorValue dv = (DiscriminatorValue) annotation;
this.relationshipName = dv.value();
// Data Management
@XmlAttribute(name="type")
protected String getRelationshipName() { return this.relationshipName; }
@XmlTransient
protected UdmPersistentObject getParent() { return this.parent; }
protected void setParent(UdmPersistentObject parent) { this.parent = parent; }
@XmlTransient
protected UdmPersistentObject getChild() { return this.child; }
protected void setChild(UdmPersistentObject child) { this.child = child; }
@PrePersist
public void merge()
Server server = (Server) SessionManager.getManager().getSession("udm-tests");
Session session = (Session)server.getActiveSession();
session.acquireUnitOfWork().registerObject(this.parent);
session.acquireUnitOfWork().registerObject(this.child);
The following is the error:
junit.framework.AssertionFailedError: Unable to create Personal Contact: [Persistent Object [com.riscs.tools.data.udm.support.PersonalContact] save failed.; nested exception is javax.persistence.EntityExistsException:
Exception Description: Cannot persist detached object [com.riscs.tools.data.udm.support.Person@1f47ae8].
Class> com.riscs.tools.data.udm.support.Person Primary Key> [403]]

Answer 1

Hello,
I wasn't clear. Merge is used on unmanaged objects to 'merge' the changes into the managed object. If the object is new, it will return a managed copy of the object. The only difference between merge and persist when passed in a new object, is that persist makes the passed in object managed, where as merge returns a managed copy - the object passed in remains unmanaged.
As for it being a no-op - it takes a unmanaged copy of an object and returns the managed copy. It will take any changes and merge them into the managed copy, which as you said, is a no-op if there aren't changes. But your object model must have changes - you set a parent-child bidirectional relationship. Also, merge will cascade over the tree. So if you call it on your parent, and you have added a new child in say a 1:M mapping, it will persist the child - if you have the relation set to cascade merge+persist.
What are the problems you run into with merge?
As for find - depending on your options, it will hit the cache. The point is that the spec requires you to only reference managed objects. I believe this is to allow the cache to maintain object identity - it would cause problems if you read back the newly persisted child and its referenced parent was unmanaged. Any changes to the parent (such as adding the child to its collection) also need to be managed, so the only way to do that is on a managed copy -either by first using find and then making the changes or by merging the changes made to a detached copy.
Best Regards,
Chris