class Default_Model_DbTable_Comment extends Zend_Db_Table_Abstract
    {
        /**
         * @var string Name of the database table
         */
        protected $_name = 'comment';

        /**
         * @desc  reference map
         *
         * Rows in the comment table are to be automatically deleted if the row in the
     * User table to which they refer is deleted
     *
     */
     protected $_referenceMap = array(
        'User' => array(
        	'columns' 		=> 'user_id',   // the foreign key(s)
    		'refTableClass' => 'Default_Model_DbTable_Users',
        	'refColumns'	=> 	'id',
        	'onDelete'		=>  self::CASCADE,
        )
    );

}
class Default_Model_DbTable_Users extends Zend_Db_Table_Abstract
{
    /**
     * @var string Name of the database table
     */
    protected $_name = 'users';


     /**
     * @desc Defining referential integrity here since we are using MyISAM
     * Dependent tables are referred via the class name.
     */
    protected $_dependentTables = 'Default_Model_DbTable_Comment';



}

class Default_Model_DbTable_Users extends Zend_Db_Table_Abstract
{
    /**
     * @var string Name of the database table
     */
    protected $_name = 'users';


     /**
     * @desc Defining referential integrity here since we are using MyISAM
     * Dependent tables are referred via the class name.
     */
    protected $_dependentTables = array('Default_Model_DbTable_Comment');



}