I have found a workaround to this problem:
I created a new field in my entity that does not exists in the database, but exists in the form.
In the validator class, I am accessing all the entity values, and also this new one that I need.
FYI, in order to access all the entity object in the validator, you need to put:
public function getTargets()
{
return self::CLASS_CONSTRAINT;
}