Mapping a tree within one table

ibisek · **Joined:** Tue Apr 08, 2008 4:50 am **Posts:** 2

Hello everyone,

I would like to ask how to do the following thing. Let's have an entity object:

Code:

@Entity
public class Category {

  @Id
  @GeneratedValue(strategy=GenerationType.AUTO)
  @Column(name = "category_id")
  private Long categoryId; //primary key
  
  @Column(name = "name")
  private String categoryName;

  @Column(name = "parent_category_id")
  private Long parentCategoryId;

  @????
  private Category parentCategory;

  @????
  private List<Category> childCategories;
}

and a corresponding underlying DB table:

Code:

CREATE TABLE categories(
  category_id BIGINT not null primary key auto_increment, 
  name VARCHAR(64), 
  parent_category_id BIGINT);

How to do the mapping for the parentCategory and childCategories fileds?

Many thanks in advance for any clues you might come up with! :)

Ibisek

roykachouh · **Posted:** Tue Apr 08, 2008 3:48 pm

You have a one to many "self-association". That is a category can have many categories as children...

So the properties of your code look ok, you will need to map the categories set using a "one-to-many" construct.

The code to manipulate the relationships might look something like this...

Code:

Category parent = new Category();
Category child = new Category();

child.setparentCategory(parent);
parent.getChildCategories().add(child);

ibisek · **Joined:** Tue Apr 08, 2008 4:50 am **Posts:** 2

So if I understand you well, I have to do it outside the entity, like for example in the DAO object, there is no way how to do it by Hibernate mapping....?

Sukirtha · **Posted:** Wed Apr 09, 2008 5:01 am

Code:

<class name="Category" table="category">
<id name="categoryId" column="category_id">
<generator class="sequence"/>
</id>
<property name="name" column="name" />
<property name="parentLocationId" column="parent_location_id" />
<set name="childCategories">
<key>
<column name="parent_category_id" />
</key>
<one-to-many class="Category" />
</set>
</class>

This is an example of self association. And I think your java bean class will change.

Code:

@Entity 
public class Category { 

  @Id 
  @GeneratedValue(strategy=GenerationType.AUTO) 
  @Column(name = "category_id") 
  private Long categoryId; //primary key 
  
  @Column(name = "name") 
  private String categoryName; 

  @Column(name = "parent_category_id") 
  private Long parentCategoryId; 

  @???? 
  private Set<Category> childCategories;

This is how I do to represent a tree structure. Maybe you can use it too.

electric monk · **Posted:** Wed Apr 09, 2008 6:54 am

I'm using this type of mapping; It works using the initial insert, but when making modifications (e.g. deleting children) and updating, I get an exception thrown.

I've just opened a thread regarding this issue here. Again, this still doesn't work completely for me, but initial insert and selects DO work, so I must be in the right direction.

Fairly straightforward, actually, just notice that you have to implement Comparable for the SortedSet to work (I'm guessing that you don't REALLY need a list).

Hope this helps.

Code:

@Entity
public class Category implements Comparable<Category>{

  @Id
  @GeneratedValue(strategy=GenerationType.AUTO)
  @Column(name = "category_id")
  private Long categoryId; //primary key
 
  @Column(name = "name")
  private String categoryName;

   @ManyToOne(fetch = FetchType.EAGER, cascade=CascadeType.ALL, optional = true)
   @JoinColumn(name = "PARENT_CATEGORY_ID", nullable = true)
   @Fetch(FetchMode.SELECT)
   @Cascade({   org.hibernate.annotations.CascadeType.ALL,
            org.hibernate.annotations.CascadeType.SAVE_UPDATE,
            org.hibernate.annotations.CascadeType.DELETE_ORPHAN})
  private Category parentCategory;

   @OneToMany(cascade = CascadeType.ALL, fetch = FetchType.EAGER, mappedBy="parentCategory")
   @Sort(type = SortType.NATURAL)
   @Fetch(FetchMode.SUBSELECT)
   @Cascade({   org.hibernate.annotations.CascadeType.ALL,
            org.hibernate.annotations.CascadeType.SAVE_UPDATE,
            org.hibernate.annotations.CascadeType.DELETE_ORPHAN})
  private SortedSet<Category> childCategories;
} 

sermojohn · **Joined:** Wed Oct 02, 2013 4:53 pm **Posts:** 1

Hello!

I have used the same mapping as proposed in the previous e-mail in order to depict the parent-child relationship. However, when I try to run

Code:

parent.getChildren()

hibernate gets in an endless loop (I waited for about 10 minutes with no result)! I also noticed that when I do the same thing for a simple parent-children hierarchy (where the parent has only one child which might have no children), there are no such issues.

Any idea what might be the problem? I checked if there are circular references, but didn't find any :)