How declare a relation Many-to-Many not required with Symfony2 and Doctrine2?

Question

I have two PHP model classes named Question and SolutionMethod. A Category may have many Items and an Item may belong to many Categories. I have created a ManyToMany relation to both classes:

class Question {
    /**
     * @ORM\ManyToMany(targetEntity="SolutionMethod", mappedBy="questions", cascade={"persist"})
     */
    private $solutions;

    /**
     * Add items
     *
     * @param Acme\MyBundle\Entity\SolutionMethod $solution
     */
    public function addItems(\Acme\MyBundle\Entity\SolutionMethod $solution) {
        $this->solutions[] = $solutions;
    }

    /**
     * Get solutions
     *
     * @return Doctrine\Common\Collections\Collection 
     */
    public function getSolutions() {
        return $this->solutions;
    }

    [...]

}

and

class SolutionMethod {
    /**
     * @ORM\ManyToMany(targetEntity="Question", inversedBy="solutions", cascade={"persist"})
     * @ORM\JoinTable(name="question_solution")
     */
    private $questions;

    /**
     * Add questions
     *
     * @param Acme\MyBundle\Entity\Question $question
     */
    public function addCategories(\Acme\MyBundle\Entity\Question $question) {
        $this->questions[] = $question;
    }

    /**
     * Get questions
     *
     * @return Doctrine\Common\Collections\Collection 
     */
    public function getQuestions() {
        return $this->questions;
    }

    [...]

}

In my form this association is rendered with a multi-select with required attribute, but I have some Question without SolutionMethod, but I can not save, and the required message appears.

Solved!

The solution is so simple to make me feel like an idiot

class QuestionType extends AbstractType {

    public function buildForm(FormBuilder $builder, array $options) {
        $builder
            ->add('solutions', null, array('required' => false))
            [...]
        ;
    }
}