Hibernate Newbie - 1 to Many Relation Tutorial

[Nakata kokuyo]

Good day All,

i'm new to hibernate and i would like to get myself understand on how 1-many relationship work in hibernate, so i need all your expertise experience contribution on the guidance

i have 2 tables(mySQL)and i assume one country have multiple contact, so assume i have one(country) to many(contact) relationship betwen tables :

CREATE TABLE contact ( 
    ID       bigint(20) NOT NULL  auto_increment ,
    FIRSTNAMEvarchar(255) NULL,
    LASTNAME varchar(255) NULL,
    EMAIL    varchar(255) NULL,
    COUNTRY_ID  varchar(255) NOT NULL,
    PRIMARY KEY(ID)
)
 

CREATE TABLE country ( 
    COUNTRY_ID      bigint(20) NOT NULL  auto_increment ,
    COUNTRY_NAMEvarchar(255) NULL,    
    PRIMARY KEY(COUNTRY_ID)
)
 

my contact.hbm.xml :

<hibernate-mapping>
  <class name="test.hibernate.test.Contact" table="Contact">
   <id name="id" type="long" column="ID" >
   <generator class="native"/>
  </id>
 
  <property name="firstName">
     <column name="FIRSTNAME" />
  </property>
  <property name="lastName">
    <column name="LASTNAME"/>
  </property>
  <property name="email">
    <column name="EMAIL"/>
  </property>
   <property name="country_id">
    <column name="COUNTRY_ID"/>
  </property>    
 </class>
</hibernate-mapping>

my question here, how to i figure out country.hbm.xml for country table ?

thanks for guidance

[Arun Kumarr]

"my question here, how to i figure out country.hbm.xml for country table ?"

- Didn't quite get what you meant?

[Nakata kokuyo]

how to i create another hbm file for country table in order to map with contact.hbm.xml to have one - many relation?

[Arun Kumarr]

Here is how you write a simple country.hbm file.

<hibernate-mapping>  <class name="test.hibernate.test.Country" table="country">   
<id name="countryId" type="long" column="COUNTRY_ID" >   
<generator class="native"/>  </id>   
 
<property name="countryName"> 
<column name="COUNTRY_NAME" />  
</property> 
 
</class>
</hibernate-mapping>

Now that you have mentioned,
[B]

country table in order to map with contact.hbm.xml to have one - many relation?

[/B]

- How do we go about doing the relationship mapping? You want it both the sides, or only one side? Have you tried using the <key-many-to-one>, <set>, <list>, <bag> tags in the hbm?

[Nakata kokuyo]

Hi Kumar,

if we go for one side , how the mapping will look like ?

[Arun Kumarr]

I see that one contact is mapped to one country and not many.
So try <one-to-one> mapping from the contact side to the country side.
Show us your xml file after that.

[Nakata kokuyo]

good day Kumar,

no doubt one contact is mapped to one country and not many, but what if one country will have many contact ? do you think is valid ?

[Nakata kokuyo]

for one to one case,

i try the following :

contact.hbm.xml

<hibernate-mapping>
  <class name="test.hibernate.test.Contact" table="Contact">
   <id name="id" type="long" column="contact_id" >
   <generator class="native"/>
  </id>
 
  <property name="firstName">
     <column name="FIRSTNAME" />
  </property>
  <property name="lastName">
    <column name="LASTNAME"/>
  </property>
  <property name="email">
    <column name="EMAIL"/>
  </property>
 

   <one-to-one
              name="id"
              class="test.Country"
              column="country_id"
              contrainted = "true"
    />
    
 </class>
</hibernate-mapping>

country.hbm.xml

<hibernate-mapping>
  <class name="test.Country" table="Country">
   <id name="id" type="string" column="country_id" >
   <generator class="native"/>
  </id>
 
  <property name="country_name" type="string"  length="255" />
 
 </class>
</hibernate-mapping>
 

it give me error like "Attribute "column" must be declared for element type "one-to-one""

[Arun Kumarr]

Check the hibernate reference document and see if there is any attribute called column for a <one-to-one> mapping.

[Nakata kokuyo]

Hi Kumar, i really cannot get it ...can you show me how would it be?

my latest try :

contact.hbm.xml

 
<hibernate-mapping auto-import="true" default-lazy="false">
  <class name="test.hibernate.test.Contact" table="Contact" >
      
   <id name="id" type="long" column="contact_id" >
   <generator class="native"/>
  </id>
 
  <property name="firstName">
     <column name="FIRSTNAME" />
  </property>
  <property name="lastName">
    <column name="LASTNAME"/>
  </property>
  <property name="email">
    <column name="EMAIL"/>
  </property>
 
  <id name="country_id" column="country_id">
            <generator class="foreign">
                <param name="property">country</param>
            </generator> 
 
       
  <one-to-one name="country" class="test.hibernate.test.Country" constrained="true" />
 
  </id>
 </class>
</hibernate-mapping>

country.hbm.xml

<hibernate-mapping>
 
    <class name="test.hibernate.test.Country" table="country">
 
    <id name="country_id" type="String" column="country_id" >
    <generator class="native"/>
    </id>
 
   <property name="country_name" type="string"  length="255" />
 
   <one-to-one
              name="contact"
              class="test.hibernate.test.Contact"
              cascade="all"
    />
 

 
 </class>
</hibernate-mapping>

getting more and more error

error code :

The content of element type "id" must match "(meta*,column*,type?,generator?)".
 

The content of element type "class" must match "(meta*,subselect?,cache?,synchronize*,comment?,tuplizer*,(id|composite-id),discriminator?,natural-id?,(version|timestamp)?,(property|many-to-one|one-to-one|component|dynamic-component|properties|any|map|set|list|bag|idbag|array|primitive-array)*,((join*,subclass*)|joined-subclass*|union-subclass*),loader?,sql-insert?,sql-update?,sql-delete?,filter*,resultset*,(query|sql-query)*)".

[Nathan Hook]

This shouldn't be this difficult.

One Contact can have One Country.

One Country can have many Contacts.

sql

CREATE TABLE contact ( 
    CONTACT_IDbigint(20) NOT NULL  auto_increment ,
    FIRSTNAMEvarchar(255) NULL,
    LASTNAME varchar(255) NULL,
    EMAIL    varchar(255) NULL,
    COUNTRY_ID  varchar(255) NOT NULL,
    PRIMARY KEY(ID)
)
 
 
CREATE TABLE country ( 
    COUNTRY_ID      bigint(20) NOT NULL  auto_increment ,
    COUNTRY_NAME    varchar(255) NULL,    
    PRIMARY KEY(COUNTRY_ID)
)

There really should be a foreign key relationship on your Contact table.


Contact.hbm.xml

<hibernate-mapping auto-import="true" default-lazy="false">
 <class name="test.hibernate.test.Contact" table="Contact" >
 
  <id name="id" type="long" column="contact_id" >
   <generator class="native"/>
  </id>
 
  <property name="firstName">
   <column name="FIRSTNAME" />
  </property>
  <property name="lastName">
   <column name="LASTNAME"/>
  </property>
  <property name="email">
   <column name="EMAIL"/>
  </property>
 
  <many-to-one name="country"
               class="test.hibernate.test.Country"
               column="country_id" />
 
 </class>
</hibernate-mapping>

Country.hbm.xml

<hibernate-mapping>
 <class name="test.hibernate.test.Country" table="country">
 
  <id name="id" 
      type="long"
      column="country_id" >
   <generator class="native"/>
  </id>
 
  <property name="country_name"
            type="string"
            length="255" />
 
  <bag name="contacts"
       table="contact"
       lazy="true"
       cascade="save-update">
        <key column="contact_id"/>
        <one-to-many class="test.hibernate.test.Contact" />
  </bag>
 
 </class>
</hibernate-mapping>

Contact.java

package test.hibernate.test;
 
public class Contact
{
  private Long aId = null;
  private String aFirstname = null;
  private String aLastname = null;
  private String aEmail = null;
  private Country aCountry = null;
 
  public Contact()
  {
 
  }
 
  public void setId(Long id)
  {
    aId = id;
  }
 
  public Long getId()
  {
    return aId;
  }
 
  public void setFirstname(String firstname)
  {
    aFirstname = firstname;
  }
 
  public String getFirstname()
  {
    return aFirstname;
  }
 
  public void setLastname(String lastname)
  {
    aLastname = lastname;
  }
 
  public String getLastname()
  {
    return aLastname;
  }
 
  public void setEmail(String email)
  {
    aEmail = email;
  }
 
  public String getEmail()
  {
    return aEmail;
  }
 
  public void setCountry(Country country)
  {
    aCountry = Country;
  }
 
  public String getCountry()
  {
    return aCountry;
  }
}

Country.java

package test.hibernate.test;
 
public class Country()
{
  private String aName = null;
  private List aContactList = null;
 
  public Country()
  {
  }
 
  public void setName(String name)
  {
    aName = name;
  }
 
  public String getName()
  {
    return aName;
  }
 
  public void setContacts(List contactList)
  {
    aContactList = contactList;
  }
 
  public List getContacts()
  {
    return aContactList;
  }
}

[ August 09, 2007: Message edited by: Nathan Hook ]

[ August 09, 2007: Message edited by: Nathan Hook ]
[ August 09, 2007: Message edited by: Nathan Hook ]

[Nakata kokuyo]

Thanks Nathan Hook, it was great example as i really confuse myself on this ....let me try out later and let you know if i have any doubts, many thanks again on this

[Nakata kokuyo]

Hi Nathan and kumar,

thanks for the sample, it work ..

i have some questions, below is my implementation :

public class FirstExample {
    public static void main(String[] args) {
        Session session = null;
 
        try {
            if (session == null) {
                System.out.println("is null.....");
            }
                        SessionFactory sessionFactory = new Configuration().configure().buildSessionFactory();
 
            session = sessionFactory.openSession();
 
            Transaction t = session.beginTransaction();
            System.out.println("Inserting Record");
 
            Country cn = new Country();
            cn.setCountry_name("JAPAN");
            session.save(cn);
 

            Contact contact = new Contact();
 
            contact.setFirstName("Deepak");
            contact.setLastName("Kumar");
            contact.setEmail("[e-mail]");
            contact.setCountry_id("JP");
            session.save(contact);
 

            t.commit();
            System.out.println("Done");
            session.flush();
 
        } catch (Exception e) {
            System.out.println(e.getMessage());
        } finally {
            // Actual contact insertion will happen at this step
 
            session.close();
 
        }
 
    }
}

the DB table result will show below :

contact_id     FIRSTNAME     LASTNAME     EMAIL               country_id    
 -------------  ------------  -----------  -------------------  ------------- 
 1              Deepak        Kumar        [e-mail](null)        

country_id     country_name    
 -------------  --------------- 
 1              JAPAN           

my question here :

<class name="test.hibernate.test.Country" table="country">
        <id name="id"       type="string"      column="country_id" >
            <generator class="native"/>
        </id>

1)if i would like to have country_id to be string , for example country_id like "JP" , and i have modify the coutry.hbm.xml as above, it prompt me error like Table server.country' doesn't exist , is it the right way to do it ?

[Nathan Hook]

I think there is a little bit of a disconnect on how/why to use Hibernate.

Hibernate is used to do Object-Relational Mapping. Meaning that Hibernate will take your object model and then persist that object model to your relational database.

So take the following example:

package test.hibernate.test;
 
public class Contact
{
  private Long aId = null;
  private String aName = null;
  private Country aCountry = null;
 
  public Contact()
  {
 
  }
 
  public void setId(Long id)
  {
    aId = id;
  }
 
  public Long getId()
  {
    return aId;
  }
 
  public void setName(String name)
  {
    aName = name;
  }
 
  public String getName()
  {
    return aName;
  }
 
  public void setCountry(Country country)
  {
    aCountry = Country;
  }
 
  public String getCountry()
  {
    return aCountry;
  }
}
 

package test.hibernate.test;
 
public class Country()
{
  private String aName = null;
  private String aCode = null;
 
  public Country()
  {
  }
 
  public void setName(String name)
  {
    aName = name;
  }
 
  public String getName()
  {
    return aName;
  }
 
  public void setCode(String code)
  {
    aCode = code;
  }
 
  public String getCode()
  {
    return aCode;
  }
}

When we're setting a Country on a User we're actually setting the entire object that we are using to represent a Country on our User.

We are not setting a specific value of that Country's value on the User.

So the database tables would like the following for storing the above classes:

CREATE TABLE contact ( 
    CONTACT_IDbigint(20) NOT NULL  auto_increment ,
    NAMEvarchar(255) NULL,
    COUNTRY_ID  varchar(255) NOT NULL,  // This is a foreign key to the country table.
    PRIMARY KEY(ID)
)
 
 
CREATE TABLE country ( 
    COUNTRY_ID      bigint(20) NOT NULL  auto_increment ,
    COUNTRY_NAME    varchar(255) NULL,    
    PRIMARY KEY(COUNTRY_ID)
)

The above are tables that are in a specific normal form. One of the reasons for using relational databases, to reduce the amount of duplicated data.

With what you're doing by placing the name of the country in the User table, is you're lowering your normal form of your database. Which is fine, but what is the purpose of the country table? Its not even being referenced and therefore there isn't any referential integrity.

My guess is you're trying to build something called a FACT table that is easy to query. If that is your case then you will need to rethink how you're using hibernate.

[Rohan Truscott]

Say that we have created a country object and have created 2 contact objects

Then we have set each contact object reference the 1 country object
i.e.

contact1 = new Contact(); // fill in object etc
contact2 = new Contact(); // fill in object etc
thecountry = new Country(); // fill in object etc
 
contact1.setCountry(thecountry);
contact2.setCountry(thecountry);

Now save all of these objects using Hibernate save() function etc...

Now how do we reconstitute (ie what does the code look like) to get this data from the database back into Java object form, like diagram below??

    
           /-- contact1
country1   |
           |
           \-- contact2

[ January 20, 2008: Message edited by: Rohan Truscott ]