New to hibernate - foreign key error

Akumadevil · **Joined:** Mon Apr 16, 2007 10:44 am **Posts:** 9

I am new to Hibernate and am having some problems. I have the following structure:

========================================

Database table: content
Primary key: int content_id
Foreign keys: int siteId (sites.site_id), int categoryId (category.category_id)

Database table: categories
Primary key: int category_id

Database table: sites
Primary key: int site_id

========================================

I have created the Java POJO's as per my database structure. The current mapping files are throwing an error because the many-to-one relationship is of type object (CategoryObject/SiteObject) but my POJO defines the foreign key as an int.

What approach should I be taking???

If I change my POJO's set/getCategoryId and set/getSiteId to objects rather than int it will not throw an error - but then how do I set these int values when saving a record to the database?

Any help would be greatly appreciated.

========================================

Hibernate version: 3.2.3

Mapping documents:

Code:

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

<hibernate-mapping package="foo.object">
    <class name="ContentObject" table="CONTENT">
        <id name="id" column="content_id">
            <generator class="native"/>
        </id>
        
        <many-to-one name="siteId" column="site_id" class="SiteObject" />
        <many-to-one name="categoryId" column="category_id" class="CategoryObject" />
    </class>
</hibernate-mapping>

Code:

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

<hibernate-mapping package="foo.object">
    <class name="CategoryObject" table="CATEGORIES">
        <id name="id" column="category_id">
            <generator class="native"/>
        </id>
        
        <property name="name" type="string" length="255" />
    </class>
</hibernate-mapping>

Code:

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

<hibernate-mapping package="foo.object">
    <class name="SiteObject" table="SITES">
        <id name="id" column="site_id">
            <generator class="native"/>
        </id>
        
        <property name="name" type="string" length="255" />
    </class>
</hibernate-mapping>

Code between sessionFactory.openSession() and session.close():

Code:

Query q = sess.createQuery("from ContentObject");
List messages = q.list();
System.out.println(messages.size() + " message(s) found:" );

Full stack trace of any exception that occurs:

Class: org.hibernate.property.BasicPropertyAccessor$BasicSetter
Timestamp: 2007-04-17 00:54:35,437
Origin: org.hibernate.property.BasicPropertyAccessor$BasicSetter.set(BasicPro
pertyAccessor.java:98)
Message: expected type: int, actual value: foo.object.SiteObject$$Enhan
cerByCGLIB$$8acb9221

Name and version of the database you are using: MySQL 4.1

batmat · **Posted:** Mon Apr 16, 2007 11:57 am

Please post your entities Java code.

ankit.zindal · **Posted:** Tue Apr 17, 2007 1:24 am

I also have the same problem.

Akumadevil · **Joined:** Mon Apr 16, 2007 10:44 am **Posts:** 9

I thought about it some more and found the solution to my problem.

Whereas before I declared my foreign keys as integers, they should in fact be declared as objects. Taking my previous code, the solution was changing my POJO from:

Code:

public class ContentObject {
   private int content_id;
   private int siteId; // Foreign key
   private int categoryId; // Foreign key

   ... (Get and Set methods)
}

to:

Code:

public class ContentObject {
   private int content_id;
   private SiteObject siteId; // Foreign key
   private CategoryObject categoryId; // Foreign key

   ... (Get and Set methods)
}

Which meant that for the way I saved a ContentObject to the database went from:

Code:

ContentObject newObj = new ContentObject();

// Save the int values
newObj.setSiteId(4);
newObj.setCategoryId(15);

sess.save(newObj); // sess is the Hibernate session

to:

Code:

ContentObject newObj = new ContentObject();
// Load the foreign key objects from the database
SiteObject siteObj = (SiteObject)sess.load(SiteObject.class, 4);
CategoryObject categoryObj = (CategoryObject)sess.load(CategoryObject.class, 15);

// Save the objects rather than the int values
newObj.setSiteId(siteObj);
newObj.setCategoryId(categoryObj);

sess.save(newObj); // sess is the Hibernate session

This will insert a new row in my content table with the integer values of 4 for site_id and 15 for category_id. No entries are made to the site or category tables unless I specify an int that doesn't exist in the foreign table.

When you think about it, handling foreign keys this way makes sense. Example: If I wanted to set siteId(999) which doesn't exist, it would first be saved to the site table. I can't just set the id to a number that doesn't exist in the site table.

I hope this helps. Please let me know if this method is not the best way, but for the moment it works.

ankit.zindal · **Posted:** Wed Apr 18, 2007 9:44 am

Hi
In your case If I m executing select query on child table it also fetches the record from the referenced parent table while we are not required data from parent table. This will make performance impact.

Also at the time of insertion/updation I have to create unneccesary object of parent type whose details I dont know.

Time being I m also doing the same but this does not looks the proper solution.