Symfony2 error with forms and related entities - forms

Question

I have been searching before for an answer for this problem but after hours I need to ask you.
I have one Entity called Pedidos, that have another entity related called PedidosMateriales, as you can see:
(...)
/**
* Pedidos
*
* #ORM\Table()
* #ORM\Entity(repositoryClass="ochodoscuatro\IntranetBundle\Entity\PedidosRepository")
*/
class Pedidos
{
/**
* #var integer
*
* #ORM\Column(name="id", type="integer")
* #ORM\Id
* #ORM\GeneratedValue(strategy="AUTO")
*/
private $id;
(...)
/**
* #ORM\OneToMany(targetEntity="PedidosMateriales", mappedBy="pedido")
*/
private $pedidosmateriales;
public function __construct() {
$this->pedidosmateriales = new \Doctrine\Common\Collections\ArrayCollection();
$this->pagos = new \Doctrine\Common\Collections\ArrayCollection();
}
public function addPedidosmateriales(\ochodoscuatro\IntranetBundle\Entity\PedidosMateriales $pedidosmateriales){
$this->pedidosmateriales[] = $pedidosmateriales;
}
public function getPedidosmateriales(){
return $this->pedidosmateriales;
}
(...)
}
When I submit a form, if I get $_POST, I can see all the information good there, but just after doing
$form->handleRequest($request);
I get this error:
Neither the property "pedidosmateriales" nor one of the methods "addPedidosmaterial()", "addPedidosmateriale()", "setPedidosmateriales()", "__set()" or "__call()" exist and have public access in class "ochodoscuatro\IntranetBundle\Entity\Pedidos".
But I have them already written!
I've read that with "multiple" => true I could get the answer, but it launch another error telling that the option "multiple" does not exist.
My form is this:
$builder->add('fechapedido', 'date', array(//Fecha del pedido
'widget' => 'single_text',
'format' => 'dd/MM/yyyy',
'attr' => array('class' => 'datepicker'),
'data' => new \DateTime() //Valor (Fecha actual)
))
->add('cliente', 'entity', array(
'class' => 'ochodoscuatroIntranetBundle:Clientes',
'property' => 'nombre',
'empty_value' => 'Nombre cliente'))//Nombre del "placeholder"
->add('pedidosmateriales', 'collection', array('type' => new PedidosMaterialesType(), 'allow_add' => true, 'by_reference' => false,));
}