Model related Form Select: Selected attribute

Hi all.

I couldn't get select options with selected attribute for my model. Simple example (I omitted some code for brevity):

<?php
// Form
class AttributeForm extends \Phalcon\Forms\Form
{
    public function initialize($entity, $options)
    {
        $group = new Select('groups[]', Attrgroup::find(), [
            'using'       => ['id', 'name'],
            'class'       => 'form-control',
            'multiple'    => 'multiple',
            'size'        => 5
        ]);

        $group->setLabel('Attribute Group');

        $this->add($group);
    }

    public function open()
    {
        $html = '';
        foreach ($this->getElements() as $element) {
            $html .= $element->render();
        }

        return $html;
    }
}

// Controller
class AttributeController extends \Phalcon\Mvc\Controller
{
    public function updateAction($id)
    {
        $model = Attribute::findFirst($id);
        $form = new AttributeForm($model, ['edit' => true]);

        if ($this->request->isPost()) {
            // validation & saving
        } else {
            $form->setEntity($model);
        }

        $this->view->setVars([
            'form'      => $form,
            'attribute' => $model
        ]);
    }
}

// Model
class Attribute extends \Phalcon\Mvc\Model
{
    public function initialize()
    {
        self::setup(['notNullValidations' => false]);

        $this->hasManyToMany(
            'id',
            __NAMESPACE__ . '\AttrgroupsAttributes',
            'attribute_id', 'group_id',
            __NAMESPACE__ . '\Attrgroup',
            'id',
            ['alias' => 'groups']
        );
    }
}

Okay. This work fine. But select options don't have selected attribute. View example:

{{ form('class': 'form-horizontal', 'role' : 'form', 'accept-charset' : 'utf-8') }}
{{ form.open() }}
{{ submit_button('Save', 'class': 'btn btn-primary') }}
{{ end_form() }}

How to assign selected attribute for related options?