symfony2 twig template load checkboxes from many to many entity relationship - symfony

Question

New Symfony2 User here. I have 2 entities that are related, one to many that is unidirectional. I'm doing it as ManyToMany as the doctrine documentation suggests, Article(one) and Tags(many). I'd like to have checkboxes show up that show the tag names on the article.new page and the article.edit page. On form submission the id of the tag entity is stored in the article_tags side table that the entity generator created for me.
Posting only relevant code.
Tag Entity AppBundle/Entity/Tag.php
/**
* #ORM\Column(name="id", type="integer")
* #ORM\Id
* #ORM\GeneratedValue(strategy="AUTO")
*/
private $id;
/**
* #var string
*
* #ORM\Column(name="name", type="string", length=20)
*/
public $name;
Article Entity AppBundle/Entity/Article.php
/**
* #ORM\ManyToMany(targetEntity="Tag")
* #ORM\JoinTable(
* name="article_tags",
* joinColumns={#ORM\JoinColumn(name="article_id", referencedColumnName="id")},
* inverseJoinColumns={#ORM\JoinColumn(name="tag_id", referencedColumnName="id", unique=true)}
* )
*/
protected $tags;
/**
* Add tag
*
* #param \AppBundle\Entity\Tag $tag
*
* #return Article
*/
public function addTag(\AppBundle\Entity\Tag $tag)
{
$this->tags[] = $tag;
return $this;
}
/**
* Remove tag
*
* #param \AppBundle\Entity\Tag $tag
*/
public function removeTag(\AppBundle\Entity\Tag $tag)
{
$this->tags->removeElement($tag);
}
/**
* Get tags
*
* #return \Doctrine\Common\Collections\Collection
*/
public function getTags()
{
return $this->tags;
}
Article Form Type AppBundle/Form/ArticleType
$builder->add('title')
->add('body')
->add('author')
->add('tags', 'entity', array(
'class' => 'AppBundle\Entity\Tag',
'property' => 'name',
'expanded' => 'true', ));
ArticleController AppBundle/Controller/ArticleController.php
* #Template()
*/
public function newAction()
{
$entity = new Article();
$tags = new Tag();
$entity->addTag($tags);
$form = $this->createCreateForm($entity);
return array('entity' => $entity,'form' => $form->createView(), );
}
As of now the error I receive is...
Entities passed to the choice field must be managed. Maybe persist
them in the entity manager?
I'm not entirely sure I'm on the right track. I just want to attach tags to articles!
Thanks

Answer 1

In the controller, you create a blank Tag and add it to the new Article before creating the form. That doesn't make sense to me, and I suspect that's where the error is coming from.
If there are any tags in the database, Symfony will automatically get them and display them with a checkbox in the form. If the user checks a checkbox, this Tag will be added to the Article.
Just delete these two lines and you should be fine:
$tags = new Tag();
$entity->addTag($tags);