fields:
…（中略）…
        donation:
            type: text
            nullable: true

    /**
     * @var string
     */
    private $donation;

    /**
     * Set donation
     *
     * @param  string $donation
     * @return Order
     */
    public function setDonation($donation)
    {
        $this->donation = $donation;

        return $this;
    }

    /**
     * Get donation
     *
     * @return string
     */
    public function getDonation()
    {
        return $this->donation;
    }

            ->add('donation', 'choice', array(
                'required' => false,
                'empty_value' => false,
                'expanded' => true,
                'choices' => array(
                    '選択肢1' => '選択肢1',
                    '選択肢2' => '選択肢2',
                    '選択肢3' => '選択肢3',
                ),
                'constraints' => array(
                    new Assert\NotBlank(array(
                        'message' => '選択してください。'
                    )),
                ),
            ));

            ->add('donation', 'choice', array(
                'required' => false,
                'empty_data' => null,
                'choices' => array(
                    '選択肢1' => '選択肢1',
                    '選択肢2' => '選択肢2',
                    '選択肢3' => '選択肢3',
                ),
            ));