これら 2 つのエラーが発生しましたが、修正方法がわかりません。私が直面している問題の 1 つは、チュートリアルの新しいバージョンと古いバージョン (Rails 4 と 3.2 の違い) の違いです。
私の仕様は次のとおりです。
ルビーのバージョン: 1.9.2p320
レールのバージョン: 3.2.13
Rスペック: 2.11.1
コンピューター: Macbook Pro OS X Mountain Lion
エラー
  1) User following and unfollowing 
     Failure/Error: before { @user.unfollow!(other_user) }
     NoMethodError:
       undefined method `find_by' for []:ActiveRecord::Relation
     # ./app/models/user.rb:36:in `unfollow!'
     # ./spec/models/user_spec.rb:47:in `block (4 levels) in <top (required)>'
  2) User following and unfollowing followed_users 
     Failure/Error: before { @user.unfollow!(other_user) }
     NoMethodError:
       undefined method `find_by' for []:ActiveRecord::Relation
     # ./app/models/user.rb:36:in `unfollow!'
     # ./spec/models/user_spec.rb:47:in `block (4 levels) in <top (required)>'
ユーザー.rb
  def following?(other_user)
    relationships.where(followed_id: other_user.id).first
  end
  def follow!(other_user)
    relationships.create!(followed_id: other_user.id)
  end
  def unfollow!(other_user)
    relationships.find_by(followed_id: other_user.id).destroy!
  end
user_spec.rb
describe "following" do
    let(:other_user) { FactoryGirl.create(:user) }
    before do
      @user.save
      @user.follow!(other_user)
    end
    it { should be_following(other_user) }
    its(:followed_users) { should include(other_user) }
    describe "followed users" do 
      subject { other_user }
      its(:followers) {should include(@user) } 
    end
    describe "and unfollowing" do
      before { @user.unfollow!(other_user) }
      it {should_not be_following(other_user) }
      its(:followed_users) {should_not include(other_user) }
    end
  end