I got troubles trying to hibernate tree like structure of data

I got class "node"

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public class node {

private Long id;
private Long parent_id;

private node parent;
private String name;
private double number;
private List array = new ArrayList();

private String depth;
private String lineage;


public node(){
this.setParent(this);
}

public void addChildren(node ch){

ch.setParent(this);
this.getArray().add(ch);

}


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

plus all setters and getters

Constructor is a bit strange because parent_id do not allows the null and one of the nodes have to be the root

and mapping file like this:

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<class name="node" table="TREE">
<id name="id" column="ID">
<generator class="increment"/>
</id>

<property name="name"/>
<property name="depth"/>
<property name="lineage"/>

<!--CHILDREN-->
<list name="array"
inverse="true"
cascade="all-delete-orphan">

<key column="parent_id" not-null="true"/>
<list-index column="number"/>
<one-to-many class="node"/>
</list>

<!--PARENTS-->
<many-to-one name="parent_id"
class="node"
column="parent_id"
not-null="true" />

</class>

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

When I try:

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Session session = HibernateUtil.currentSession();
Transaction tx = session.beginTransaction();

node node1 = new node();
node node2 = new node();
node node3 = new node();

node1.setName("first");
node2.setName("second");
node3.setName("3rd");

node2.addChildren(node3);
node1.addChildren(node2);


session.save(node1);

tx.commit();
HibernateUtil.closeSession();
------------------------------------------------

I got:

16:00:29,979 INFO SchemaExport:173 - schema export complete
16:00:29,979 INFO DriverManagerConnectionProvider:147 - cleaning up connection pool: jdbc:mysql://localhost/tutorial
16:00:29,979 INFO SessionFactoryImpl:379 - Checking 0 named queries
Exception in thread "main" org.hibernate.PropertyValueException: not-null property references a null or transient value: data.object.node.parent_id
at org.hibernate.engine.Nullability.checkNullability(Nullability.java:72)
at org.hibernate.event.def.AbstractSaveEventListener.performSaveOrReplicate(AbstractSaveEventListener.java:236)
at org.hibernate.event.def.AbstractSaveEventListener.performSave(AbstractSaveEventListener.java:160)
at org.hibernate.event.def.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:108)
at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.saveWithGeneratedOrRequestedId(DefaultSaveOrUpdateEventListener.java:184)
at org.hibernate.event.def.DefaultSaveEventListener.saveWithGeneratedOrRequestedId(DefaultSaveEventListener.java:33)
at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.entityIsTransient(DefaultSaveOrUpdateEventListener.java:173)
at org.hibernate.event.def.DefaultSaveEventListener.performSaveOrUpdate(DefaultSaveEventListener.java:27)
at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.onSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:69)
at org.hibernate.impl.SessionImpl.save(SessionImpl.java:481)
at org.hibernate.impl.SessionImpl.save(SessionImpl.java:476)
at data.object.node.main(node.java:63)

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

Do you have any suggestions what is the problem?


Sincerely,

Pawel

The problem arises from the fact that all your nodes are nodes!! You have a not-null constraint on parent_id, so your root node must have a root node. Quick solutions:

1- remove the not-null constraint and monitor the values by java code
2- create a root_node_class and map it somehow without not-null constraints

Bye.

Well, Hibernate rightfully complains about parent_id being null because H does not-null check before persisting an object.

Is there a problem to use ‘assigned’ IDs – it is slightly less convenient but perhaps will help to avoid this problem.

As a side note: it looks like you are trying to let Hibernate to take care of entire tree structure. I do not think this is a good idea. I suggest looking at ‘nested set’ tree model from Joe Celko’s SQL for Smarties (and I definitely recommend the book) http://www.dbmsmag.com/9603d06.html

That model is really much more convenient to deal with in RDBMS, especially for hierarchical and roll-up queries.