例如：我想添加一篇博客，里面有一个分类，我应该有一个category_id字段，然后在添加博客的时候可以通过下拉列表选择。

疑问：
我的下拉列表中怎么才能有我添加好的分类列表呢！
posts和category的关系

 #Demo\TestBundle\Entity\Posts
Demo\TestBundle\Entity\Posts:
type: entity
table: posts
repositoryClass: Demo\TestBundle\Repository\PostsRepository
id:
    id:
        type: integer
        id: true
        genertor: { strategy: AUTO }
fields:
    title: 
        type: string
        length: 64
    author:
        type: string
        length: 32
    create_at:
        type: integer
    update_at:
        type: integer
manyToOne:
    category:
        targetEntity: Category
        inversedBy: posts


Demo\TestBundle\Entity\Category:
type: entity
table: category
repositoryClass: Demo\TestBundle\Repository\CategoryRepository
id:
    id:
        type: integer
        id: true
        genertor: 
            strategy: AUTO
fields:
    title: 
        type: string
        length: 16
    create_at:
        type: integer
    update_at:
        type: integer
oneToMany:
    posts:
        targetEntity: Posts
        mappedBy: category

form表单时通过命令生成的:php app/console doctrine:generate:form DemoTestBundle:Posts

 <?php

namespace Demo\TestBundle\Form;

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolverInterface;

class PostsType extends AbstractType
{
    /**
 * @param FormBuilderInterface $builder
 * @param array $options
 */
public function buildForm(FormBuilderInterface $builder, array $options)
{
    $builder
        ->add('id')
        ->add('title')
        ->add('author')
        ->add('create_at')
        ->add('update_at')
        ->add('category')
    ;
}

/**
 * @param OptionsResolverInterface $resolver
 */
public function setDefaultOptions(OptionsResolverInterface $resolver)
{
    $resolver->setDefaults(array(
        'data_class' => 'Demo\TestBundle\Entity\Posts'
    ));
}

/**
 * 表单标示符， name属性
 * @return string
 */
public function getName()
{
    return 'posts';
}

}

大概就是这样子，不知道哪里要调还是代码错误了，希望跟路大侠指教。