Scoped uniqueness validation of associated objects

class Poll < ActiveRecord::Base
has_many :answers

validates_associated :answers

accepts_nested_attributes_for :answers, :allow_destroy => true end

class Answer < ActiveRecord::Base
belongs_to :poll

validates_uniqueness_of :body, :scope => :poll_id end

When new poll is created,
attributes for associated answers are passed,
but uniqueness of answers body in scope of poll is not validated.

Any ideas how to fix that.