Symfony2 submit error with entity type (many-many relationship)

I have created in Symfony2 a simple Many to Many ($ post -> $ category) relation that I want to use in my form builder thanks to the "entity" field type, with multiple = false (default) for now, although that's a lot for many.

This is part of my Post object:

 /**
 * @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 am getting

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 relationship is many to many, so I am forced to make this field with multiple objects?

I cleared cache, doctrine cache, I updated my getter / setter object, dont know what I might be doing wrong.

thanks for the help