Carrierwave`default_url`没有触发-Java 学习之路

我正在尝试让carrierwave为没有上传任何图像的记录显示默认图像 .

我正在使用carrierwave默认代码 . 在app / uploaders / image_uploader.rb下：

def default_url
  ActionController::Base.helpers.asset_path("fallback/" + [version_name, "default.png"].compact.join('_'))
end

我将我的默认图像“default.png”存储在assets / images / fallback中 .

在我的视图中，我正在使用图像助手标记：<％= image_tag @ listing.images.first.url％>注意： @listing 是记录的实例

对于具有图像的记录，它显示没有问题 . 对于没有图像的记录，我得到一个错误“未定义的方法`url'代表nil：NilClass” .

在byebug放置在视图中：

对于具有图像的记录： @record.images 返回其中包含图像文件名的数组 .
对于没有图像的记录： @record.images 返回一个空数组 .

当byebug置于 default_url 时，当没有图像的记录被访问时，它根本不被触发 .

[完整代码]
应用程序/上传/ image_uploader.rb：

class ImageUploader < CarrierWave::Uploader::Base

# Include RMagick or MiniMagick support:
  include CarrierWave::RMagick
  # include CarrierWave::MiniMagick

  # Choose what kind of storage to use for this uploader:
  # storage :file
  storage :fog

  # Override the directory where uploaded files will be stored.
  # This is a sensible default for uploaders that are meant to be mounted:
  def store_dir
    "uploads/#{model.class.to_s.underscore}/#{mounted_as}/#{model.id}"
  end

  # Provide a default URL as a default if there hasn't been a file uploaded:
  def default_url(*args)
    # For Rails 3.1+ asset pipeline compatibility:
    ActionController::Base.helpers.asset_path("fallback/" + [version_name, "default.png"].compact.join('_'))

    # "/fallback/" + [version_name, "default.png"].compact.join('_')
  end

  # Process files as they are uploaded:
  # process scale: [200, 300]
  #
  # def scale(width, height)
  #   # do something
  # end

  # Create different versions of your uploaded files:
  version :thumb do
    process resize_to_limit: [200, 200]
  end

  # Add a white list of extensions which are allowed to be uploaded.
  # For images you might use something like this:
  def extension_whitelist
    %w(jpg jpeg gif png)
  end

  # Override the filename of the uploaded files:
  # Avoid using model.id or version_name here, see uploader/store.rb for details.
  # def filename
  #   "something.jpg" if original_filename
  # end

end

Carrierwave`default_url`没有触发

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