如何验证列可以更新一次?

我有一个协议模型,它有一个 released_at 列。我想验证协议是否可以发布 曾经。 我该怎么做?

控制器:

 # GET /aggreements/1/release
 def release
   @agreement.update(released_at: Time.now.utc)

   if @agreement.save
     render json: {success: ["Agreement released."]}
   else
     render json: @agreement.errors, status: :unprocessable_entity
   end
 end    

模型:

class Agreement < ApplicationRecord
  validate :released_agreement_cannot_be_released

  def released_agreement_cannot_be_released
    if released_at.present?
      errors.add(:released_at, "already released")
    end
  end
end

谢谢您。