Persist a graph with JPA - jpa

Question

I try to persist a graph with JPA, but it doesn't work. I have Leaf(s) which are Node(s), which can have multiple Parent(s) and a Parent can have multiple children (Node).
#Entity
#Table(name = "NODES")
#Inheritance(strategy = InheritanceType.JOINED)
#DiscriminatorColumn(name = "KIND", discriminatorType = DiscriminatorType.STRING, length = 1)
public abstract class Node implements Serializable {
#Column(unique = true, name = "NAME", nullable = false, length = 50)
private String name;
#ManyToMany(mappedBy = "children", fetch = FetchType.LAZY)
#JoinTable(name = "NODE_PARENTS", joinColumns = { #JoinColumn(name = "NODEID", referencedColumnName = "ID") }, inverseJoinColumns = { #JoinColumn(name = "PARENTID", referencedColumnName = "ID") })
private Set<Parent> parents;
public Node() {
this.parents = new HashSet<Parent>();
}
public String getName() {
return this.name;
}
public void setName(String name) {
this.name = name;
}
public void setParents(Collection<Parent> parentList) {
for(Parent parent : this.parents) {
removeParent(parent);
}
for (Parent parent : parentList) {
addParent(parent);
}
}
public void addParent(Parent parent) {
this.parents.add(parent);
if (!parent.getMembers().contains(this)) {
parent.getMembers().add(this);
}
}
public void removeParent(ACLGroup parent) {
this.parents.remove(parent);
if (parent.getMembers().contains(this)) {
parent.getMembers().remove(this);
}
}
public Set<ACLGroup> getParents() {
return Collections.unmodifiableSet(this.parents);
}
}
#Entity
#Table(name = "LEAFS")
#DiscriminatorValue("L")
public class Leaf extends Node {
// some fields to persist
}
#Entity
#Table(name = "INNER_NODES")
#DiscriminatorValue("P")
public class InnerNodes extends Node implements Parent {
// some fields to persist
#ManyToMany(fetch = FetchType.LAZY)
private Set<Node> children;
public InnerNodes() {
this.children = new HashSet<Node>();
}
public Set<Node> getChildren() {
return Collections.unmodifiableSet(this.children);
}
public void setChildren(Set<Node> childList) {
for (Node child : this.children) {
removeChild(child);
}
for (Node child : childList) {
addChild(child);
}
}
public void addChild(Node child) {
this.children.add(child);
if (!child.getParents().contains(this)) {
child.addParent(this);
}
}
public void removeChild(Node child) {
this.children.remove(child);
if (child.getParents().contains(this)) {
child.removeParent(this);
}
}
}
public interface Parent {
Set<Node> getChildren();
void setChildren(Set<Node> childList);
void addChild(Node child);
void removeChild(Node child);
}
The problem is, I can not persist the interface Parent, but I need it to prevent circles. Every Leaf or InnerNode can have multiple Parents and every Parent can have multiple children.
Any idea to implement that?
Thanks a lot.
Andre