Question

I’m having a trouble with a class that’s associated with itself. The object it’s this:

Category.java

package com.borjabares.pan_ssh.model.category;

import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;
import javax.persistence.SequenceGenerator;

import com.borjabares.pan_ssh.util.Trimmer;

@Entity
public class Category {
private long categoryId;
private String name;
private Category parent;

public Category() {
}

public Category(String name, Category parent) {
    this.name = name;
    this.parent = parent;
}

@SequenceGenerator(name = "CategoryIdGenerator", sequenceName = "CategorySeq")
@Id
@GeneratedValue(strategy = GenerationType.AUTO, generator = "CategoryIdGenerator")
public long getCategoryId() {
    return categoryId;
}

public void setCategoryId(long categoryId) {
    this.categoryId = categoryId;
}

public String getName() {
    return name;
}

public void setName(String name) {
    this.name = Trimmer.trim(name);
}

@ManyToOne(optional=false, fetch=FetchType.EAGER) 
@JoinColumn(name="categoryId", insertable=false, updatable=false, nullable = true)
public Category getParent() {
    return parent;
}

public void setParent(Category parent) {
    this.parent = parent;
}

@Override
public String toString() {
    return "Category [\ncategoryId=" + categoryId + ", \nname=" + name
            + ", \nparent=" + parent + "]";
}
}

The association it’s only one level deep. Insert, and other queries are working. But when I try to select only parent categories or non parent categories Hibernate only returns 0 results for parent categories or all the results of the table.

The queries, as they are right now is like this, but I’ve a lot of other queries with joins, is null and other methods obtaining always the same result.

@SuppressWarnings("unchecked")
public List<Category> listParentCategories() {
    return getSession().createQuery(
            "SELECT c FROM Category c WHERE c.parent is null ORDER BY c.name")
            .list();
}

Thank you, and sorry for the mistakes I’ve maybe made writing this.

EDIT:

Insertion works fine, when I list all the categories in jUnit and print them I’ve got this:

Category [
categoryId=416, 
name=Deportes, 
parent=null], 
Category [
categoryId=417, 
name=Formula 1, 
parent=Category [
categoryId=416, 
name=Deportes, 
parent=null]], 
Category [
categoryId=418, 
name=F?tbol, 
parent=Category [
categoryId=416, 
name=Deportes, 
parent=null]]

Besides in the insertion I’m controlling that a category can only be parent or child, and a category can’t be his own father.

Answer 1

You’re using the same column (categoryId) to uniquely identify a category, and to reference the parent category. This can’t possibly work, since all categories would obviously have themselves as parent. You need another column to hold the parent category ID:

@Id
@SequenceGenerator(name = "CategoryIdGenerator", sequenceName = "CategorySeq")
@GeneratedValue(strategy = GenerationType.AUTO, generator = "CategoryIdGenerator")
@Column(name = "categoryId") // not necessary, but makes things clearer
public long getCategoryId() {
    return categoryId;
}

// optional must be true: some categories don't have a parent
@ManyToOne(optional = true, fetch = FetchType.EAGER) 
// insertable at least must be true if you want to create categories with a parent
@JoinColumn(name = "parentCategoryId", nullable = true)
public Category getParent() {
    return parent;
}