Question

    I am getting the following error I am new to ruby and rails

    I am using Rails4 with eclipse plugin the error is 
    NoMethodError in Books#show 

    undefined method `name' for nil:NilClass

    Extracted source (around line #3):  
    @book.subject.name I used associations in the model book which is belongs_to association with subject model.
    <h1><%= @book.title %></h1>
    <p><strong>Price: </strong> $<%= @book.price %><br />
    <strong>Subject: </strong> <%= link_to @book.subject.name,    #line number 3
    :action => "show_subjects", :id => @book.subject.id %><br />
    <strong>Created Date:</strong> <%= @book.created_at %><br />
    </p>

    app/views/books/show.erb:3:in `_app_views_books_show_erb___1047429498_14766720'
    actionpack (4.0.3) lib/action_view/template.rb:143:in `block in render'
    activesupport (4.0.3) lib/active_support/notifications.rb:161:in `instrument'
    actionpack (4.0.3) lib/action_view/template.rb:141:in `render'

    routes.rb file is

    LibrarayWebApplication::Application.routes.draw do
        get 'books/list'
        get 'books/new'
        post 'books/create'
        post 'books/update'
      get 'books/list'
       get 'books/show'
       get 'books/show_subjects'
      get 'books/edit'
        get 'books/delete'
          get 'books/update'
     get 'books/show_subjects'
    end

    my migrations files are

    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
      t.column :created_at, :timestamp
         end
      end

      def self.down
        drop_table :books
      end
    end

    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

    my controller files is

    class BooksController < ApplicationController
       def list
          @books = Book.find(:all)
       end
       def show
          @book = Book.find(params[:id])
       end
       def new
          @book = Book.new
          @subjects = Subject.find(:all)
       end
      #  private 
       def book_params
       params.require(:book).permit(:title,:price,:subject,:description)
       end
       def create
          @book = Book.new(book_params)
          if @book.save
                redirect_to :action => 'list'
          else
                @subjects = Subject.find(:all)
                render :action => 'new'
          end
       end


          def edit
          @book = Book.find(params[:id])
          @subjects = Subject.find(:all)
       end
      def update
            @book = Book.find(params[:id])

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

    show.erb file is
    I used associations in the model book which is belongs_to association with subject model.
    <h1><%= @book.title %></h1>
    <p><strong>Price: </strong> $<%= @book.price %><br />
        <strong>Subject: </strong> <%= link_to @book.subject.name, #this line I am getting error
    :action => "show_subjects", :id => @book.subject.id %><br />
    <strong>Created Date:</strong> <%= @book.created_at %><br />
    </p>
    <p><%= @book.description %></p>
    <hr />
    <%= link_to 'Back', {:action => 'list'} %>

    show_subjects file is

    <h1><%= @subject.name -%></h1>
    <ul>
    <% @subject.books.each do |c| %>
    <li><%= link_to c.title, :action => "show", :id => c.id -%></li>
    <% end %>
    </ul>

    I have two models 
    book.erb

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

I made associations between the models in book model I used belongs_to and in subject model I used has_many but I am getting that error If I remove the subject in show.erb the application working fine but when I include that line it showing that no such method

我在书籍模型中使用了belongs_to的模型之间建立了关联，在主题模型中我使用了has_many，但是我收到了错误如果我在show.erb中删除主题，应用程序工作正常，但当我包含该行时，它显示了没有这样的方法我在书模型中使用belongs_to和模型之间建立了关联我使用has_many但我得到了错误如果我在show.erb中删除主题应用程序工作正常但是当我包含该行时它显示没有这样的方法

    subject.erb
    class Subject < ActiveRecord::Base
        has_many :books
    end

我写了@ book.subject.name如果@ book.subject这句话避免了错误，但它无法从数据库中检索主题名称

Answer 1

您收到的是未定义的方法错误，因为您没有该特定图书的主题。换句话说，您应该在实际调用book.subject.name之前检查是否存在图书主题。

P.S。我会改变您的主题迁移（抛弃Subject.create＆＃39; ...＆＃39;部分）并使用资源：路线的书籍，而不是单独定义所有路线。

Answer 2

在show.html.erb而不是@book.subject.name中写为@book.subject.try(:name)或@book.subject.name if @book.subject

书籍中的NoMethodError＃show

2 个答案: