As ofsymfony/symfony#3239 the "collection" and "choice" types (with multiple=true) as well as subtypes (such as "entity") will try to discover an adder and a remover method in your modelif "by_reference" is set to false.

Example form:

$form = $this->createFormBuilder($article)    ->add('tags', null, array('by_reference' => false))    ->getForm();

The underlying article class is expected to look like this:

class Article{    public function addTag($tag) { ... }    public function removeTag($tag) { ... }    public function getTags($tag) { ... }}

If adder and remover are not found, the form behaves differently depending on whethergetTags returns an object or an array.

1. If it returns an object (collection), the object is modified by reference and not written back
2. If it returns an array, the array is modified and written back usingsetTags

You can force a write-back in case (1) by setting the field option "by_reference" to false. Note that in this case, adders and removerswon't be used even if present! Instead, the expected object signature is

class Article{    public function setTags($tags) { ... }    public function getTags($tag) { ... }}