想利用 gravater 为用户提供一个默认的头像。希望通过 carrierwave 从 Gravatar 获取头像，并做一个缩小后的版本保存在本地。我试图在 user before_create 中做处理，但是失败了，avatar 中依然为空。以下是代码...

class User
  mount_uploader :avatar, AvatarUploader

  before_create do |user|
    gravatar_url = Gravatar.new(user.email).image_url
    user.avatar = open(gravatar_url)
  end
end

class AvatarUploader < CarrierWave::Uploader::Base

  include CarrierWave::MiniMagick
  storage :file
  def store_dir
    "upload/#{model.class.to_s.underscore}/#{mounted_as}/#{model.id}"
  end
  def scale(width, height)
    process resize_to_fit: [64, 64]
  end
  version :tiny do
    process resize_to_fit: [24, nil]
  end
  def extension_white_list
    %w(jpg jpeg gif png)
  end

end