I have a form that combines two entities (User and Profile).
Validation seems to work on the first part of the form that comes form the User Entity and is the basis
I was looking for the exact same thing and here is what I found
http://symfony.com/doc/master/book/forms.html#forms-embedding-single-object
You need to tell the main entity to validate it's sub entities like so:
/**
* @Assert\Type(type="AppBundle\Entity\Category")
* @Assert\Valid()
*/
private $subentity;
I've tested this on symfony 2.8 and it works.