->add('Category', ChoiceType::class, [
                'choice_label' => 'Name',
                'multiple' => true,
                'mapped' => false,
                'expanded' => true,
                'choices' => $this->categoryRepository->getList(null, true),
                'choice_value' => function (Category $Category = null) {
                    return $Category ? $Category->getId() : null;
                },

                 // ★追加する
                'constraints' => [
                  new Assert\Count([
                     'min' => 2,
                  ]),
                ],
            ])