使用rspec测试无表格模型

Question

我想测试没有相应表格的模型。这是模型：

class ContactMessage

  include ActiveModel::Validations
  include ActiveModel::Conversion
  extend ActiveModel::Naming

  attr_accessor :sender_name, :sender_email, :subject, :content, :recipient_name, :recipient_email

  validates_presence_of :sender_name
  validates_format_of :sender_email, :with => /^[-a-z0-9_+\.]+\@([-a-z0-9]+\.)+[a-z0-9]{2,}$/i
  validates_presence_of :subject
  validates_length_of :content, :minimum => 10

  def initialize(attributes = {})
    attributes.each do |name, value|
      send("#{name}=", value)
    end
  end

  def persisted?
    false
  end

  def sender
    sender_name + " <" + sender_email + ">"
  end

  def recipient
    if recipient_name
      recipient_name + " <" + recipient_email + ">"
    else
      recipient_email
    end
  end

end

...但是当我尝试在其上运行标准模型测试时，例如：

it { should validate_presence_of :sender_email }

......我得到了回报：

undefined method `reflect_on_association' for ContactMessage:Class

如何设置这些测试？