显示SELECT列表值为SELECT from DB - Joomla 2.5

我有一个Multi SELECT List，管理员可以为球员选择多个位置，球员可以玩

/com/models/forms/player.xml

 <field name="sec_position_name" type="checkboxes" label="Secondary Pos." >
        <option value="Left">Left</option>
        <option value="Middle">Middle</option>
        <option value="Right">Right</option>
 </field>

这些是

com/models/player.php加载表单数据

protected function loadFormData() {
    $data = JFactory::getApplication()->
            getUserState('com_football.edit.player.data', array());
    if (empty($data)) {
        $data = $this->getItem();
    }
    return $data;
}
public function getForm($data = array(), $loadData = true) {
    $form = $this->loadForm('com_football.player', 
                            'player', array('control' => 'jform',
                            'load_data' => $loadData));        
    return $form;
}

下面是表单在布局中的显示方式。

com/views/player/tmpl/edit.php

<ul class="adminformlist">
<?php foreach ($this->form->getFieldset('optional') as $field): ?>
   <li><?php echo $field->label; ?>
   <?php echo $field->input; ?></li>
<?php endforeach ?>
</ul>

我如何从DB显示已经存在的值作为SELECTED…??

注意:我在这里也问过(也许这更清楚)