Unable to create a model with composite primary key

Hi,

I have three tables in Mysql DB.

1. user: _id as PK
2. role: _id as PK
3. user_role: _id as AI user_id FK to user role_id FK to role PRIMARY KEY(user_id, role_id)

Models:

User

$this->hasMany('_id', 'UserRole', 'user_id', array('alias' => 'UserRole'));

Role

$this->hasMany('_id', 'UserRole', 'role_id', array('alias' => 'UserRole'));

UserRole

1. $this->belongsTo('role_id', 'Role', '_id', array('alias' => 'Role'));
2. $this->belongsTo('user_id', 'User', '_id', array('alias' => 'User'));

$model = new UserRole();
$model->user_id =1;
$model->role_id = 1;
$model->create();
/*
Which throws error:
Phalcon\Mvc\Model\Message Object
(
    [_type:protected] => InvalidCreateAttempt
    [_message:protected] => Record cannot be created because it already exists
    [_field:protected] => 
    [_model:protected] => 
)
*/

NOTE: user_role table is empty

Can any one plz suggest to overcome this error and how to define the relations in my above scenario.

thanks in advance.