->add(
                'quantity',
                'integer',
                array(
                    'required' => false,
                    'mapped' => true,
                    'empty_data' => null,
                    'constraints' => array(
                        new Assert\NotBlank(),
                        new Assert\Range(
                            array(
                                'min' => 1,
                                'max' => 100,
                            )
                        ),
                    ),
                )
            )