Hibernate3: Self-Referencing Objects

Question

Need some help on understanding how to do this; I'm going to be running recursive 'find' on a file system and I want to keep the information in a single DB table - with a self-referencing hierarchial structure:

This is my DB Table structure I want to populate.

DirObject Table:

id       int NOT NULL,
name     varchar(255) NOT NULL,
parentid int NOT NULL);

Here is the proposed Java Class I want to map (Fields only shown):

public DirObject {
    int id;
    String name;
    DirObject parent;
...

For the 'root' directory was going to use parentid=0; real ids will start at 1, and ideally I want hibernate to autogenerate the ids.

Can somebody provide a suggested mapping file for this please; as a secondary question I thought about doing the Java Class like this instead:

public DirObject {
    int id;
    String name;
    List<DirObject> subdirs;

Could I use the same data model for either of these two methods ? (With a different mapping file of course).

--- UPDATE: so I tried the mapping file suggested below (thanks!), repeated here for reference:

<hibernate-mapping>
    <class name="my.proj.DirObject" table="category">
        ...   

        <set name="subDirs" lazy="true" inverse="true">
            <key column="parentId"/>
            <one-to-many class="my.proj.DirObject"/>
        </set>

        <many-to-one name="parent"
                     class="my.proj.DirObject"
                     column="parentId" cascade="all" />
    </class>

...and altered my Java class to have BOTH 'parentid' and 'getSubDirs' [returning a 'HashSet'].

This appears to work - thanks, but this is the test code I used to drive this - I think I'm not doing something right here, because I thought Hibernate would take care of saving the subordinate objects in the Set without me having to do this explicitly ?

DirObject dirobject=new DirObject();
   dirobject.setName("/files");
   dirobject.setParent(dirobject);

   DirObject d1, d2;
   d1=new DirObject(); d1.setName("subdir1"); d1.setParent(dirobject);
   d2=new DirObject(); d2.setName("subdir2"); d2.setParent(dirobject);
   HashSet<DirObject> subdirs=new HashSet<DirObject>();
   subdirs.add(d1);
   subdirs.add(d2);
   dirobject.setSubdirs(subdirs);


   session.save(dirobject);
   session.save(d1);
   session.save(d2);

Answer 1

I believe this will work ... completely untested.

<hibernate-mapping>
    <class name="my.proj.DirObject" table="category">
        ...   

        <set name="subDirs" lazy="true" inverse="true">
            <key column="parentId"/>
            <one-to-many class="my.proj.DirObject"/>
        </set>

        <many-to-one name="parent"
                     class="my.proj.DirObject"
                     column="parentId" cascade="all" />
    </class>
</hibernate-mapping>

Answer 2

you can get the children from parent

<set name="subdirs" lazy="false" cascade="all-delete-orphan" inverse="true">
            <key column="parentid " />
            <one-to-many class="DirObject" />
 </set>

parent from child

<many-to-one name="parent" class="DirObject">
            <column name="parentid" />
 </many-to-one>

Answer 3

You can actually have the following Java entity:

public DirObject {
    int id;
    String name;
    DirObject parent;
    List<DirObject> subdirs;
    ...
}

And map it on the DIROBJECT table:

ID       int NOT NULL,
NAME     varchar(255) NOT NULL,
PARENTID int NOT NULL);

Using the following mapping:

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"&gt;

<hibernate-mapping package="mypackage">

  <class name="DirObject" table="DIROBJECT">

    <id name="id" type="int">
      <column name="ID" />
      <generator class="native" />
    </id>

    <property name="name" type="string">
      <column name="NAME" length="150" not-null="true" unique="false" index="NAME" />
    </property>

    <bag name="subdirs" lazy="false" cascade="all-delete-orphan" inverse="true">
      <key column="PARENTID" />
      <one-to-many class="DirObject" />
    </bag>

    <many-to-one name="parent" class="DirObject">
      <column name="PARENTID" />
    </many-to-one>
  </class>

</hibernate-mapping>

Answer 4

This is really helpful. I was trying to figure out a similar solution using "Annotations". How would I do that?

Thank you.