将索引与Presenter一起使用会导致NoMethodError

Question

我试着学习演示者，并完成了以下课程

带有id、first_name、last_name、电子邮件的用户模型

user_controller，使用以下方法

def index
@users = User.all
render_user_json @users
end
def show
@user = User.find_by_id(params[:id])
if @user.nil?
render json: { message: 'User can not be found' }, status: 404
return
end
render_user_json @user
end
def render_user_json(user)
render json: ::Presenters::User::UserPresenter.new(user).generate
end

UserPresenter类

module Presenters
module User
class UserPresenter
attr_reader :user
def initialize(user)
@user = user
end
def generate
{
id: @user.id,
first_name: @user.first_name,
last_name: @user.last_name,
email: @user.email
}
end
end
end
end

当我转到localhost:3000/users/id时，它运行良好但是当我转到localhost:3000/users时，我会得到以下错误

NoMethodError (undefined method `id' for #<User::ActiveRecord_Relation:0x00007f9952ac4930>
Did you mean?  ids):
lib/presenters/user/user_presenter.rb:14:in `generate'
app/controllers/users_controller.rb:49:in `render_user_json'
app/controllers/users_controller.rb:9:in `index'

Rendered /Users/noammansur/.rvm/gems/ruby-2.3.8/gems/actionpack-4.2.11/lib/action_dispatch/middleware/templates/rescues/_source.erb (4.0ms)
Rendered /Users/noammansur/.rvm/gems/ruby-2.3.8/gems/actionpack-4.2.11/lib/action_dispatch/middleware/templates/rescues/_trace.html.erb (2.0ms)
Rendered /Users/noammansur/.rvm/gems/ruby-2.3.8/gems/actionpack-4.2.11/lib/action_dispatch/middleware/templates/rescues/_request_and_response.html.erb (1.0ms)
Rendered /Users/noammansur/.rvm/gems/ruby-2.3.8/gems/actionpack-4.2.11/lib/action_dispatch/middleware/templates/rescues/diagnostics.html.erb within rescues/layout (108.3ms)
Rendered /Users/noammansur/.rvm/gems/ruby-2.3.8/gems/web-console-2.3.0/lib/web_console/templates/_markup.html.erb (0.4ms)
Rendered /Users/noammansur/.rvm/gems/ruby-2.3.8/gems/web-console-2.3.0/lib/web_console/templates/_inner_console_markup.html.erb within layouts/inlined_string (0.3ms)
Rendered /Users/noammansur/.rvm/gems/ruby-2.3.8/gems/web-console-2.3.0/lib/web_console/templates/_prompt_box_markup.html.erb within layouts/inlined_string (0.3ms)
Rendered /Users/noammansur/.rvm/gems/ruby-2.3.8/gems/web-console-2.3.0/lib/web_console/templates/style.css.erb within layouts/inlined_string (0.4ms)
Rendered /Users/noammansur/.rvm/gems/ruby-2.3.8/gems/web-console-2.3.0/lib/web_console/templates/console.js.erb within layouts/javascript (91.6ms)
Rendered /Users/noammansur/.rvm/gems/ruby-2.3.8/gems/web-console-2.3.0/lib/web_console/templates/main.js.erb within layouts/javascript (0.7ms)
Rendered /Users/noammansur/.rvm/gems/ruby-2.3.8/gems/web-console-2.3.0/lib/web_console/templates/error_page.js.erb within layouts/javascript (0.8ms)
Rendered /Users/noammansur/.rvm/gems/ruby-2.3.8/gems/web-console-2.3.0/lib/web_console/templates/index.html.erb (210.6ms)

它似乎没有通过单一用户

Answer 1

而不是这个：

render_user_json @users

要么制作另一个方法render_users_json(注意复数用户(，要么直接从index方法调用render，如下所示：

# inside index method
render json: users.map do |user|
::Presenters::User::UserPresenter.new(user).generate
end