Expected value of type "Customize\Entity\Example" for association field "Eccube\Entity\Order#$example_id", got "integer" instead.

<?php

namespace Customize\Entity;

use Doctrine\ORM\Mapping as ORM;

use Symfony\Bridge\Doctrine\Validator\Constraints\UniqueEntity;
use Symfony\Component\Validator\Mapping\ClassMetadata;

if (!class_exists('\Eccube\Entity\Example')) {
    /**
     * Shop
     *
     * @ORM\Table(name="dtb_example")
     * @ORM\InheritanceType("SINGLE_TABLE")
     * @ORM\DiscriminatorColumn(name="discriminator_type", type="string", length=255)
     * @ORM\HasLifecycleCallbacks()
     * @ORM\Entity(repositoryClass="Customize\Repository\ExampleRepository")
     */
    class Example extends \Eccube\Entity\AbstractEntity
    {
        /**
         * @ORM\Column(name="id", type="integer", options={"unsigned":true})
         * @ORM\Id
         * @ORM\GeneratedValue(strategy="IDENTITY")
         */
        private $id;

        /**
         * @ORM\Column(name="name", type="string", length=255, nullable=false)
         */
        private $name;

        /**
         * Get id.
         * @return int
         */
        public function getId()
        {
            return $this->id;
        }

        /**
         * Set name.
         * @param string $name
         * @return Example
         */
        public function setName($name)
        {
            $this->name = $name;
            return $this;
        }

        /**
         * Get name.
         * @return string
         */
        public function getName()
        {
            return $this->name;
        }
    }
}

<?php

namespace Customize\Entity;

use Doctrine\ORM\Mapping as ORM;
use Eccube\Annotation as Eccube;
use Symfony\Component\Validator\Constraints as Assert;

/**
 * @Eccube\EntityExtension("Eccube\Entity\Order")
 */
trait OrderTrait
{
    /**
     * @var \Customize\Entity\Example
     * @ORM\ManyToOne(targetEntity="Customize\Entity\Example")
     * @ORM\JoinColumns({
     *   @ORM\JoinColumn(name="example_id", referencedColumnName="id", nullable=true)
     * })
     * @Assert\NotBlank
     * @Eccube\FormAppend(
     *  auto_render=true,
     *  type="Symfony\Component\Form\Extension\Core\Type\ChoiceType",
     *  options={
     *  "eccube_form_options":{
     *    "auto_render":true
     *  },
     *  "choices":{
     *    "選択肢１":1,
     *    "選択肢２":2,
     *  },
     *  "label":"テスト",
     *  "required":true,
     *  "expanded":true,
     *  "multiple":false,
     *  "data":1,
     *  })
     */
    private $example_id;

    /**
     * @param  integer  $example_id
     * @return Order
     */
    public function setExampleId($example_id = 1)
    {
        $this->example_id = $example_id;
        return $this;
    }

    /**
     * @return integer
     */
    public function getExampleId()
    {
        return $this->example_id;
    }
}