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problem implementing many-to-many relationship

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vaibhavmishra

Post subject: problem implementing many-to-many relationship

Posted: Thu Feb 09, 2006 6:54 am

Newbie

Joined: Thu Feb 09, 2006 5:23 am
Posts: 1

Hi,

I am using Hibernate 3.0 & SQL Server 2000.

I have 3 tables Account (Id[PK], Name) , Role (Id[PK], Description), R_Account_Role (Account_Id[FK], Role_Id[FK]). Account and Role have a many-to-many relationship via R_Account_Role table.

Account.hbm.xml

Code:

<hibernate-mapping>
    <class name="Account" table="Account">
        <id name="id" column="ID" type="integer" >
            <generator class="identity"/>
        </id>
        <property name="name" column="Name" not-null="true" type="string" />
        <set name="roles" table="R_Account_Role" lazy="false" cascade="all" >
            <key column="Account_Id" not-null="true"/>
            <many-to-many column="Role_Id" class="Role" />
        </set>
    </class>
</hibernate-mapping>

Role.hbm.xml

Code:

<hibernate-mapping>
    <class name="Role" table="Role">
        <id name="id" column="ID" type="integer" >
            <generator class="identity"/>
        </id>
        <property name="description" column="Description" not-null="true" type="string" />
        <set name="accounts" table="R_Account_Role" lazy="false" cascade="all" >
            <key column="Role_Id" not-null="true"/>
            <many-to-many column="Account_Id" class="Account" />
        </set>
    </class>
</hibernate-mapping>

Java code to delete an account

Code:

Account account = (Account) session.load(Account.class, accountId);
session.delete(account);
session.flush();

If I execute this, then all the roles associated with this account also gets deleted from Role table.
Ideally, only the records from Account-Role relationship table should get deleted.

If I do not use "cascade=all" then exception is thrown when I try to delete Account, since accountId is foreign key in R_Account_Role table.

Basically, I want to be able to create, update, delete an Account (and records in Account-Role relationship table), keeping Role table relatively constant [i.e. number of roles are fixed]

I am confused whether to create another hbm.xml file for R_Account_Role table or above code is sufficient with some changes. Please help.

Thanx.

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bdn

Post subject:

Posted: Thu Feb 09, 2006 8:51 am

Newbie

Joined: Thu Feb 09, 2006 4:07 am
Posts: 3

There probably is an elegant solution. See others for that.

As a temporary workaround (I cannot try it out but you could): what about first removing all the roles (buy assigning it an empty Set or perhaps even simply null) and subsequently removing the account?

Code:

Account account = (Account) session.load(Account.class, accountId);
account.setRoles(new HashSet());     // first remove all roles for this account
session.delete(account);             // and next delete the account
session.flush();

Moreover, I notice that you've set the non-null attribute to "true" on the key of the associated entity. You may ask yourself if that's really required (in my implementation of many-to-many relationships this isn't the case and everything works fine).

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