Phalcon模型关系..ToMany（）不起作用

I tried to setup relationships between models Users and Groups via Users_Group using different methods, with and without namespace, with hasManyToMany and hasMany etc. nothing works and gives me error: Fatal error: Class 'Users_Groups' not found ... Here is my UserController portion:

    public function registerAction()
{
    $user = new Users();
    $password = $this->security->hash($this->request->getPost('password'));
    $signup = array(
        'name'=>$this->request->getPost('name'),
        'username'=>$this->request->getPost('username'),
        'email'=>$this->request->getPost('email'),
        'password'=>$password,
        'active'=>'Y',
        'datetime'=>date('Y-m-d H:i:s', time())
        );
    //Store and check for errors
    $success = $user->save($signup, array('name', 'username', 'email', 'password', 'active', 'datetime'));
    $group = Groups::findByName('user');
    $userGroup = new Users_Groups();
    $userGroup->user_id = $user->getId();
    $userGroup->group_id = $group->getId();
    $groupSuccess = $userGroup->save();
    if ($success && $groupSuccess) {
        echo "Thanks for registering!";
    } else {
        echo "Sorry, the following problems were generated: ";
        foreach ($user->getMessages() as $message) {
            echo $message->getMessage(), "<br/>";
        }
    }

    $this->view->disable();

}

Here is Users model:

    public function initialize()
{
    $this->hasMany(
        'id',
        'Users_Groups',
        'user_id'
        );

    $this->hasMany('id','Qsos','user_id');

    $this->hasMany('id','Configs','user_id');
}

Here is Groups model:

    public function initialize()
{
    $this->hasMany(
        'id',
        'Users_Groups',
        'group_id'
        );
}

And finally Users_Groups model

    public function initialize()
{
    $this->belongsTo('user_id','Users','id');
    $this->belongsTo('group_id','Groups','id');
}

Please help! It drives me crazy!