Ruby on Rails - nil:NilClass 的未定义方法“错误”答案

【问题标题】：Ruby on Rails - undefined method `errors' for nil:NilClassRuby on Rails - nil:NilClass 的未定义方法“错误”
【发布时间】：2016-10-22 12:10:12
【问题描述】：

这些问题似乎重复，但我检查了所有其他相关问题，但没有一个答案适用，因为我已经尝试了所有这些问题。

我正在尝试创建用户注册表单。

我的用户模型（user.rb）是这样的。

class User < ApplicationRecord
  attr_accessor :password
  EMAIL_REGEX = /^[A-Z0-9._%+-]+@[A-Z0-9.-]+\.[A-Z]{2,4}$/i
  validates :username, :presence => true, :uniqueness => true, :length => { :in => 4..20 }
  validates :email, :presence => true, :uniqueness => true, :format => EMAIL_REGEX
  validates :password, :confirmation => true #password_confirmation attr
  validates_length_of :password, :in => 6..20, :on => :create
end

用户控制器（users_controller.rb）是这样的

class UsersController < ApplicationController

  def new
    @user = User.new
  end

  def create

    @user = User.new(user_params)

    if @user.save
        flash[:notice] = "You have signed up successdully"
        flash[:color] = "valid"
    else
        flash[:notice] = "Invalid Input!"
        flash[:color] = "invalid"
    end
  end

    def user_params
      params.require(:username).permit(:email, :password, :password_confirmation)
    end     

    render "new"

end

而用户的新视图（new.html.erb）是这样的。

<% @page_title = "UserAuth | Signup" %>
<div class="Sign_Form">
  <h1>Sign Up</h1>
  <%= form_for(:user, :url => {:controller => 'users', :action => 'create'}) do |f| %>
    <p> Username:</br> <%= f.text_field :username%> </p>
    <p> Email:</br> <%= f.text_field :email%> </p>
    <p> Password:</br> <%= f.password_field :password%></p>
    <p> Password Confirmation:</br> <%= f.password_field :password_confirmation%> </p>
    <%= f.submit :Signup %>
  <% end %>
  <% if @user.errors.any? %>
    <ul class="Signup_Errors">
    <% for message_error in @user.errors.full_messages %>
      <li>* <%= message_error %></li>
    <% end %>
    </ul>
  <% end %>
</div>

路由（routes.rb）是这样的

Rails.application.routes.draw do
  get 'users/new'
  resources :users

  # For details on the DSL available within this file, see http://guides.rubyonrails.org/routing.html
  root 'home#index'
end

并且 rake route 给了我以下输出。

   Prefix Verb   URI Pattern               Controller#Action
users_new GET    /users/new(.:format)      users#new
    users GET    /users(.:format)          users#index
          POST   /users(.:format)          users#create
 new_user GET    /users/new(.:format)      users#new
edit_user GET    /users/:id/edit(.:format) users#edit
     user GET    /users/:id(.:format)      users#show
          PATCH  /users/:id(.:format)      users#update
          PUT    /users/:id(.:format)      users#update
          DELETE /users/:id(.:format)      users#destroy
     root GET    /                         home#index

然而，当我尝试从浏览器访问 http://localhost/users/new 时，出现以下错误。

Showing C:/Sites/expenses/app/views/users/new.html.erb where line #11 raised:

undefined method `errors' for nil:NilClass
Extracted source (around line #11):
9
10
11
12
13
14

    <%= f.submit :Signup %>
  <% end %>
  <% if @user.errors.any? %>
    <ul class="Signup_Errors">
    <% for message_error in @user.errors.full_messages %>
      <li>* <%= message_error %></li>

【问题讨论】：

标签： ruby-on-rails ruby

【解决方案1】：

Render new 应该在 create 的错误处。

这意味着，如果您在创建操作中有任何错误，@user 对象将连同这些错误一起被扔回新表单

因此，您可以使用 @user.errors.any? 循环遍历这些错误

 class UsersController < ApplicationController

  def new
    @user = User.new
  end

  def create

    @user = User.new(user_params)

    if @user.save
        flash[:notice] = "You have signed up successdully"
        flash[:color] = "valid"
    else
        flash[:notice] = "Invalid Input!"
        flash[:color] = "invalid"
        render "new"
    end
  end

    def user_params
      params.require(:username).permit(:email, :password, :password_confirmation)
    end     
end

【讨论】：