ruby-on-rails – “未定义的方法`验证’”错误 – RoR Tut 5.11

错误消息：

NoMethodError in PostsController#new
undefined method `  validates' for #<Class:0x0000000414fab0>

Extracted source (around line #2):
1 class Post < ActiveRecord::Base
2     validates :title,presence: true,3                       length: { minimum: 5 }
4 end

我的帖子.rb：

class Post < ActiveRecord::Base
  validates :title,length: { minimum: 5 }
end

我的posts_controller.rb：

class PostsController < ApplicationController

  def index
    @posts = Post.all 
  end

  def new
    @post = Post.new
  end

  def create
    @post = Post.new(params[:post].permit(:title,:text))

    if @post.save
      redirect_to @post
    else
      render 'new'
    end
  end

  def show
    @post = Post.find(params[:id])
  end

  private

  def post_params
    params.require(:post).permit(:title,:text)
  end
end

Sidenote – FWIW,在尝试诊断时,我从posts_controller.rb中删除了@ post = Post.new,并在刷新时出现此错误：

undefined method `errors' for nil:NilClass
Extracted source (around line #3):

    <h1>New Post</h1>
    <%= form_for :post,url: posts_path do |f| %>
    <% if @post.errors.any? %>
      <div id="error_explanation">
        <h2><%= pluralize(@post.errors.count,"error") %> prohibited this post from being saved:</h2>
      <ul>

找不到new.html.erb部分的任何问题,所以我转向社区寻求指导.谁知道我哪里出错了？在此先感谢您的帮助.