gistfile1.php

<?php
class MyFormFilter extends BaseMyFormFilter{

	public function configure(){
		$this->widgetSchema['areas_conhecimento'] = 
			new sfWidgetFormChoice(array( 
				'expanded' => true,
				'choices'  => Doctrine_Core::getTable('MyTable')->getAreasConhecimento(),
			));
	}

	public function getFields(){
		$fields = parent::getFields();
		$fields['areas_conhecimento'] = 'ForeignKey';
		return $fields; 
	}