ErrorException: Catchable Fatal Error: Object of class could not be converted to string - Caused by dropdown menu but why?

I have the following code, which retrieves the page slugs from the database which are needed to then create a related sub page:

        $builder->add('subtocontentoptions', 'entity', array(
            'class' => 'ShoutAdminBundle:Content',
            'property' => 'slug',
            'query_builder' => function($repository) { 
                return $repository->createQueryBuilder('p')
                ->where('p.mainpage = :main')
                ->setParameter('main', '1')
                ->orderBy('p.created', 'ASC');
            }
        ));

The code works, as it displays a drop down menu of all the parent pages I have. However, when I go to save the data to the database, I am given the following error:

ErrorException: Catchable Fatal Error: Object of class Shout\AdminBundle\Entity\Content could not be converted to string in C:\wamp\www\vendor\doctrine-dbal\lib\Doctrine\DBAL\Statement.php line 131

I have checked the contents of the Content entity file, and here is the variable being declared:

/**
 开发者_高级运维* @var integer $subtocontentoptions
 *
 * @ORM\Column(name="SubToContentOptions", type="integer", nullable=false)
 */
private $subtocontentoptions;

And lower down the Content entity file:

/**
 * Set subtocontentoptions
 *
 * @param integer $subtocontentoptions
 */
public function setSubtocontentoptions($subtocontentoptions)
{
    $this->subtocontentoptions = $subtocontentoptions;
}

/**
 * Get subtocontentoptions
 *
 * @return integer 
 */
public function getSubtocontentoptions()
{
    return $this->subtocontentoptions;
}

The rest of the code does work, once this drop down has been taken out. I'm not sure why the drop down is causing this error?

Thanks

---

Was having the same issue with a sf2/doctrine2 project, implementing the __toString method resolved this issue for me :

public function __toString()
{
    return strval($this->id);
}