NameError：未初始化的常量Shoulda

我执行本教程https://www.digitalocean.com/community/tutorials/build-a-restful-json-api-with-rails-5-part-one

但是，在“模型”一章中运行RSpec测试时，出现以下错误。

C:\Users\NCH-Lap10\Desktop\rubyprojects\todos-api>bundle exec rspec

An error occurred while loading ./spec/controllers/items_controller_spec.rb.
Failure/Error:
  Shoulda::Matchers.configure do |config|
    config.integrate do |with|
      with.test_framework :rspec
      with.library :rails
    end
  end

NameError:
  uninitialized constant Shoulda
# ./spec/rails_helper.rb:6:in `<top (required)>'
# ./spec/controllers/items_controller_spec.rb:1:in `require'
# ./spec/controllers/items_controller_spec.rb:1:in `<top (required)>'

An error occurred while loading ./spec/controllers/todos_controller_spec.rb.
Failure/Error:
  Shoulda::Matchers.configure do |config|
    config.integrate do |with|
      with.test_framework :rspec
      with.library :rails
    end
  end

NameError:
  uninitialized constant Shoulda
# ./spec/rails_helper.rb:6:in `<top (required)>'
# ./spec/controllers/todos_controller_spec.rb:1:in `require'
# ./spec/controllers/todos_controller_spec.rb:1:in `<top (required)>'

An error occurred while loading ./spec/models/item_spec.rb.

因此他找不到我的测试。（我从教程中复制了它们）

我的测试位于规格/模型中，如下所示：

require 'rails_helper'

# Test suite for the Item model
RSpec.describe Item, type: :model do
  # Association test
  # ensure an item record belongs to a single todo record
  it { should belong_to(:todo) }
  # Validation test
  # ensure column name is present before saving
  it { should validate_presence_of(:name) }
end

我的Gemfile的一部分：

group :test do
  gem 'factory_bot_rails', '~> 4.0'
  gem 'shoulda-matchers', '~> 3.1'
  gem 'faker'
  gem 'database_cleaner'
end

我应该提供更多代码吗？