ZF2 and Doctrine input filters for new, and existing entities

Im learning doctrine and i have a form with ZF2 and doctrine with field "email".
This field need to be unique, so i need validator for it. Problem is, when i use:

DoctrineModule\Validator\UniqueObject

it is impossible to create new entity. This validator need primary key to compare.
Validator dump error with message:

Expected context to contain itemId

itemId is my primary key.

So its obvious, that i need to use UniqueObject for update, and:

DoctrineModule\Validator\NoObjectExists

for new entity. And question is:

What is the best way, to store different input filter specification for existing and new entity?

Or, better if possible: How to use Unique validator with new, and existing records with zend form fieldsets.

If i put it in the form, I need to modify it inside controller if entity is new or no. Not so good idea.

I think that the best way is to store input filter spec. inside entity repository, but how i can check there if entity is new or not?

---- edit

I saw documentation, i know how to use unique object but i have error like said before: "Expected context to contain itemId". I think problem is with fieldsets (im using it). I dont understand how to do this (text from docs):

If you leave out the use_context option or set it to false you have to
pass an array containing the fields- and identifier-values into
isValid(). When using Zend\Form this behaviour is needed if you're
using fieldsets.

Ok im using fieldsets, so now what i can do? How i can pass correct values to isValid when im using zend forms?