How to make a custom form collection choice field with Symfony 2.6?

I have an entity named Devis with a oneToMany relation with an entity DevisMainsDoeuvre and this one contains a ManyToOne relation with the Entity MainDoeuvre like this:

Devis 1---∞ DevisMainsDoeuvre ∞-----1 MainDoeuvre

I need to fill the entity DevisMainsDoeuvre from The entity devis with a choice field. So to do that, i need to create a custom collection choice field. But my problem is that i don't know how to mix arraycollection with a choice field in a custom form type. The documentation is not very detailed for this: http://ift.tt/1mhbyqV

Any help will be appreciate.