Symfony2错误提交表单与实体类型（多对多关系）

I created a simple Many to Many ($post --> $category) relation in symfony2, which I want to use in my form builder thanks to the "entity" field type, with a multiple = false option (default) for now, eventhough it's many to many.

This is part of my Post entity :

 /**
 * @var ArrayCollection
 *
 * @ORM\ManyToMany(targetEntity="Yeomi\PostBundle\Entity\Category", inversedBy="posts")
 */
private $categories;



/**
 * Add categories
 *
 * @param \Yeomi\PostBundle\Entity\Category $categories
 * @return Post
 */
public function addCategory(\Yeomi\PostBundle\Entity\Category $category)
{
    $this->categories[] = $category;
    return $this;
}

/**
 * Remove categories
 *
 * @param \Yeomi\PostBundle\Entity\Category $categories
 */
public function removeCategory(\Yeomi\PostBundle\Entity\Category $category)
{
    $this->categories->removeElement($category);
}


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

This is my PostType :

public function buildForm(FormBuilderInterface $builder, array $options)
{
    $builder
        ->add('title', 'text')
        ->add('content', 'textarea')
        ->add('published', 'checkbox')
        ->add("categories", "entity", array(
            "class" => "YeomiPostBundle:Category",
            "property" => "name",
            "multiple" => false,
        ))
        ->add('published', 'checkbox')
        ->add('save', "submit")
    ;
}

And this is the error I get

500 Internal Server Error - NoSuchPropertyException

Neither the property "categories" nor one of the methods "addCategory()"/"removeCategory()", "setCategories()", "categories()", "__set()" or "__call()" exist and have public access in class "Yeomi\PostBundle\Entity\Post". 

It works with multiple = true, but that's not what I want, would it be because the relation is many to many so I am forced to make it a multiple entity field ?

I've cleared cache, doctrine cache, I've regenerate my entity getter/setter, don't know what I may be doing wrong..

Thanks for your help

Found the answer thanks to symfony2 Entity select tag display

The issue was indeed because of the ManyToMany relation, (this actually makes sense, because if you have a ManyToMany relation, you want to be able to select multiple entities),

You can get this to work with a multiple = false by adding a setter for the whole ArrayCollection property.

/**
 * Set categories
 * @param \Doctrine\Common\Collections\Collection $categories
 *
 * @return Post
 */
public function setCategories($categories)
{

    if(!is_array($categories))
    {
        $categories = array($categories);
    }
    $this->categories = $categories;

    return $this;
}