Question

我正在尝试关注this tutorial。它已经在Rails的早期版本中编写，我正在使用Rails 4.有一个下拉列表，选择没有出现，我收到以下错误：

NoMethodError in Book#show
Showing C:/Ruby193/mylibrary/app/views/book/show.html where line #3 raised:

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

 1 <h1><%= @book.title %></h1>
 2 <p><strong>Price: </strong> $<%= @book.price %><br />
 3 <strong>Subject: </strong> <%= @subject.name %><br />
 4 </p>
 5 <p><%= @book.description %></p>
 6 <hr />

Rails.root: C:/Ruby193/mylibrary

这是show.html

  <h1><%= @book.title %></h1>
    <p><strong>Price: </strong> $<%= @book.price %><br />
    <strong>Subject: </strong> <%= @book.subject.name %><br />
    </p>
    <p><%= @book.description %></p>
    <hr />
    <%= link_to 'Back', {:action => 'list'} %>

这是migrate / 258412_subjects.rb

class Subjects < ActiveRecord::Migration
  def self.up
      create_table :subjects do |t|
       t.column :name, :string
    end
    Subject.create :name => "Physics"
    Subject.create :name => "Mathematics"
    Subject.create :name => "Chemistry"
    Subject.create :name => "Psychology"
    Subject.create :name => "Geography"
  end

  def self.down
      drop_table :subjects
  end
end

这是migrate / 05465_books.rb

class Books < ActiveRecord::Migration
  def self.up
     create_table :books do |t|
    t.column :title, :string, :limit => 32, :null => false
    t.column :price, :float
    t.column :subject_id, :integer
    t.column :description, :text
     end
  end

  def self.down
    drop_table :books
  end
end

这是models / book.rb

class Book < ActiveRecord::Base
    belongs_to :subject
    validates_presence_of :title
    validates_numericality_of :price, :message=>"Error Message"
end

这是我的控制器类book_controller.rb

class BookController < ApplicationController

  def list
    @books = Book.all
  end
  def show
    @book = Book.find(params[:id])
  end
  def new
    @book = Book.new
    @subjects = Subject.all
  end
  def create
    @book = Book.new(book_params)
    if @book.save!
      redirect_to :action => 'list'
    else
      @subjects = Subject.all
      render :action => 'new'
    end
  end
  def edit
    @book = Book.find(params[:id])
    @subjects = Subject.all
  end
  def update
    @book = Book.find(params[:id])
    if @book.update_attributes(book_params)
      redirect_to :action => 'show', :id => @book
    else
      @subjects = Subject.all
      render :action => 'edit'
    end
  end
  def delete
    Book.find(params[:id]).destroy
    redirect_to :action => 'list'
  end

  private

  def book_params
    params.permit(:title, :price, :description)
  end

end

我该怎么办？，提前谢谢

Answer 1

根据您的错误

未定义的方法`name＆＃39;为零：NilClass

Extracted source (around line #3):

 1 <h1><%= @book.title %></h1>
 2 <p><strong>Price: </strong> $<%= @book.price %><br />
 3 <strong>Subject: </strong> <%= @subject.name %><br />

@subject在你的节目中是零。由于您的控制器已

def show
  @book = Book.find(params[:id])
end

对于你的节目，似乎是这种情况。

您为节目发布的代码的行为

<strong>Subject: </strong> <%= @book.subject.name %><br />

我建议您尝试重新启动服务器，因为它似乎已缓存了旧版本的视图。

Answer 2

你应该写

@book.subject.name

当然，您应该测试是否设置了subject。一种方法是编写

@book.subject.try(:name)

如果name不是零，则会尝试subject。

一般性评论：不要在迁移中播种数据库。通常，您只会迁移一次，但是当您进入生产阶段或进行测试时，您将运行rake db:setup，这将创建数据库架构而不运行迁移。

而是使用db/seed.rb为数据库设定种子。这是一个可重复的过程，在运行测试时（从干净/空的数据库开始）也需要这个过程。

Ruby Rails - NoMethodError，下拉列表

2 个答案: