ько что перешел на использование ActiveStorage на рельсах 5.1.4, и я новичок в TDD и пытаюсь выяснить, как протестировать модель, котораяhas_one_attached :avatar

require 'rails_helper'

RSpec.describe User, :type => :model do

  let (:valid_user) { FactoryBot.build(:user) }
  describe "Upload avatar" do
    context "with a valid image" do      
      it "saves the image" do
        valid_user.save!        
        saved_file = valid_user.avatar.attach(io: File.open("/home/ubuntu/workspace/spec/fixtures/files/avatar.jpg"), filename: "face.jpg", content_type: "image/jpg")
        expect(saved_file).to be_an_instance_of(ActiveStorage::Attachment::One)
      end
    end
  end 

end

Но я получаю следующую ошибку:

Failures:

  1) User Upload avatar with a valid image saves the image
     Failure/Error:
       saved_file = valid_user.avatar.attach(io: File.open("/home/ubuntu/workspace/spec/fixtures/files/avatar.jpg"), filename: "face.jpg", 
                                             content_type: "image/jpg")


 NoMethodError:
   undefined method `upload' for nil:NilClass
   Did you mean?  load
 # /usr/local/rvm/gems/ruby-2.3.4/gems/activestorage-0.1/lib/active_storage/blob.rb:48:in `upload'
 # /usr/local/rvm/gems/ruby-2.3.4/gems/activestorage-0.1/lib/active_storage/blob.rb:21:in `block in build_after_upload'
 # /usr/local/rvm/gems/ruby-2.3.4/gems/activestorage-0.1/lib/active_storage/blob.rb:16:in `tap'
 # /usr/local/rvm/gems/ruby-2.3.4/gems/activestorage-0.1/lib/active_storage/blob.rb:16:in `build_after_upload'
 # /usr/local/rvm/gems/ruby-2.3.4/gems/activestorage-0.1/lib/active_storage/blob.rb:26:in `create_after_upload!'
 # /usr/local/rvm/gems/ruby-2.3.4/gems/activestorage-0.1/lib/active_storage/attached.rb:25:in `create_blob_from'
 # /usr/local/rvm/gems/ruby-2.3.4/gems/activestorage-0.1/lib/active_storage/attached/one.rb:9:in `attach'
 # ./spec/models/user_spec.rb:47:in `block (4 levels) in <top (required)>'

Есть намеки?