Validates data of other model when accepts_nested_attributes_for is used

I have belongs to and has many relation.

The problem is that I need to check if score field from nested attributes is less 0, it should not save records instead return to form with error message.

The code I am trying is:

class QuizAttempt < ActiveRecord::Base

  has_many :quiz_answers
  accepts_nested_attributes_for :quiz_answers, reject_if: :invalid_score

  validate :score_greater_than_zero

  private

  def score_greater_than_zero
    errors.add(:quiz_answers, 'must greater than 0') if invalid_score(attributes)
  end

  def invalid_score(attributes)
    attributes['score'].to_i > 0
  end
end


class QuizAnswer < ActiveRecord::Base
  belongs_to :quiz_attempt
end

The form field looks like that containing array:

name="quiz_attempt[quiz_answers_attributes][5][score]"