Ruby on rails 操作邮件程序附件的未定义方法URL

我得到的错误是：

ActionView::Template::Error (undefined method `url' for nil:NilClass)

我的附件是零

<%= image_tag attachments['tippedlogods.png'].url %>

在my production.rb中：

  config.action_mailer.raise_delivery_errors = true
  config.action_mailer.delivery_method = :smtp
  config.action_mailer.default_url_options = { :host => 'hidden-peak-xxxx.heroku.com', :only_path => false}
  config.action_mailer.asset_host = 'hidden-peak-xxxx.heroku.com'
  ActionMailer::Base.smtp_settings = {
  :address    => "smtp.sendgrid.net",
  :port       => 587,
  :user_name  => ENV['SENDGRID_USERNAME'],
  :password   => ENV['SENDGRID_PASSWORD'],
  :domain     => ENV['SENDGRID_DOMAIN'],
  :authentication  => :plain
  }

这不是在确认电子邮件中发送图像的方式吗

我在跟随RoR指南

---

注意Rails 4~

在添加附件之前，您首先要呼叫

邮件。因此，在呈现模板时没有附件


更改语句的顺序

attachments.inline['tippedlogods.png'] = File.read("app/assets/images/tippedlogods.png")
mail(to: @order.email, subject: "Order Confirmation", content_type: "text/html")

链接到Rails 3。也许有什么改变了？以下是相关章节：
attachments.inline['tippedlogods.png'] = File.read("app/assets/images/tippedlogods.png")
mail(to: @order.email, subject: "Order Confirmation", content_type: "text/html")