I think the best is to have a many to many relationship between your entities and to add a link to your target entity in your "formType", you can call the query builder to filter your options for checkboxes (here "Active Tags"). This Class will work for add and edit
Here is an example with 2 Entities : Product and Tag
Entity/Product
/**
* @ORM\ManyToMany(targetEntity="Name\NameBundle\Entity\Tag",cascade={"persist"})
* @ORM\JoinTable(name="product_tags")
*/
private $tags;
Form/ProductType
->add('tags','entity',array("label"=>" your tags",'attr'=>array('class'=>'form-control'),
'class'=>"Name\NameBundle\Entity\Tag",
'property'=>'name',
'multiple'=>true,
'expanded'=>true,
'required'=>false,
'query_builder'=>function(ER $er){
return $er->getActiveTags();
}))