Join Tables with values

aarnott50 · **Joined:** Fri Feb 05, 2010 12:18 pm **Posts:** 2

I'm really new to Hibernate and I've run into some issues as I've been monkeying around with it.

I have these tables:

Code:

CREATE TABLE attribute (
  ATTRIBUTE_ID bigint(20) unsigned NOT NULL AUTO_INCREMENT,
  `NAME` varchar(64) NOT NULL,
  PRIMARY KEY (ATTRIBUTE_ID)
) ENGINE=MyISAM  DEFAULT CHARSET=latin1;

CREATE TABLE car (
  CAR_ID bigint(20) unsigned NOT NULL AUTO_INCREMENT,
  `NAME` varchar(64) NOT NULL,
  PRIMARY KEY (CAR_ID)
) ENGINE=MyISAM  DEFAULT CHARSET=latin1;

CREATE TABLE car_attribute (
  CAR_ID bigint(20) unsigned NOT NULL,
  ATTRIBUTE_ID bigint(20) unsigned NOT NULL,
  `VALUE` text NOT NULL,
  KEY CAR_ID (CAR_ID)
) ENGINE=MyISAM DEFAULT CHARSET=latin1;

And I have an Object to represent the Car and an Object to represent an Attribute. Each car can have many attributes. How do I define the relationship in my Car object so that it has attributes that draw their values from the car_attribute table?

Here is my current Car.hbm.xml (before I added the VALUE column to car_attribute):

Code:

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

    <class name="Car" table="CAR">
        <id name="id" column="CAR_ID">
            <generator class="native"/>
        </id>
        <property name="name"/>
        
        <set name="attributes" table="CAR_ATTRIBUTE">
           <key column="CAR_ID"/>
           <many-to-many column="ATTRIBUTE_ID" class="Attribute"/>
       </set>
        
    </class>

</hibernate-mapping>

Thanks,

Andrew

Froestel · **Posted:** Fri Feb 05, 2010 12:42 pm

Hi aarnott50,

I suppose you as well have entity classes corresponding to the tables you presented. So I'd imagine that the collection of attributes should be of a type CarAttribute rather than Attribute since the latter holds no values. The Attribute entity looks more like a name provider and is used as a field in CarAttribute, right?

CU
Froestel

aarnott50 · **Joined:** Fri Feb 05, 2010 12:18 pm **Posts:** 2

Thanks for the quick reply!

As it is right now, I just have a Car class and an Attribute class. What I'm looking to do is the following:

If I have a Car named 'Ferrari', it could have one of several attributes: color, mph, etc. I could also have another car named 'Junker' that just doesn't have some of these attributes recorded (maybe nobody has ever tested the mph of Junker).

So Attribute represents the type of attribute the car has and the VALUE column in the join table represents the value of that particular attribute. Are you suggesting that in my Object model I should have a Car object that has a set of CarAttribute objects that each have an Attribute object? I'd prefer not to have to represent the join table with an intermediary object.

Odds are, however, I'm just missing your suggestion because I still have a pretty weak understanding of how hibernate works. Are there any places you could suggest a very basic set of tutorials (I find the provided documentation to be a bit above my level of understanding at this point)? Maybe a book geared to new users?

Froestel · **Posted:** Fri Feb 05, 2010 1:26 pm

Hi aarnott50,

I don't think you will need to have an intermediate object to represent the relation. If you want to stick to a two class model you need to have the value be part of the Attribute class. If there is need you could restrict the values for the name of Attribute in a different way.
I suppose you don't need to be ending up (at least not necessarily) with an n:m-relationship between Car and Attribute. I could imagine that you can reduce it to be a 1:n-relationship, i.e. a Car has many Attributes. This could possibly lead to duplicate attributes but the object size is not that large, isn't it? (You don't want to jump into some big issue right away with n:m, keep it simple for a start and if you get acquainted you are soon up for more)

For a Hibernate start you probably want to check out Cameron Wallace McKenzie's book and site.

CU
Froestel