nil的未定义方法`map':NilClass,是什么原因造成的?

问题描述 投票:6回答:3

嗨我正面临这个错误,对rails来说是全新的,所以无法弄清楚是什么导致它

我的newBook.html.erb

<html>
    <head>
        <title> new Book </title>
    </head>
    <body>
        <h1><%= @hello_message %></h1>
        <h1>Add new book</h1>
        <%= form_tag :action => 'create' %>
        <p>
            <label for="book_title">Title</label>:
            <%= text_field 'book', 'title' %>
        </p>
        <p>
            <label for="book_price">Price</label>:
            <%= text_field 'book', 'price' %>
        </p>
        <p>
            <label for="book_subject">Subject</label>:
            <%= collection_select(:book,:subject_id,@subjects,:id,:name) %>
        </p>
        <p>
            <label for="book_description">Description</label>
            <br/>
            <%= text_area 'book', 'description' %>
        </p>
        <%= submit_tag "Create" %>
        <%= end_form_tag %>
        <%= link_to 'Back', {:action => 'list'} %>
    </body>
</html>

我的书模型:book.rb

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

我的主题模型:subject.rb

class Subject < ActiveRecord::Base
  attr_accessible :name
  has_many :book

end

堆栈跟踪是:

actionpack (3.2.13) lib/action_view/helpers/form_options_helper.rb:364:in `options_from_collection_for_select'
actionpack (3.2.13) lib/action_view/helpers/form_options_helper.rb:600:in `to_collection_select_tag'
actionpack (3.2.13) lib/action_view/helpers/form_options_helper.rb:191:in `collection_select'
app/views/home/newBook.html.erb:19:in `_app_views_home_new_ook_html_erb__299261930_24178164'
actionpack (3.2.13) lib/action_view/template.rb:145:in `block in render'
activesupport (3.2.13) lib/active_support/notifications.rb:125:in `instrument'
actionpack (3.2.13) lib/action_view/template.rb:143:in `render'
# -- snipped --
ruby-on-rails
3个回答
19
投票
<%= collection_select(:book,:subject_id,@subjects,:id,:name) %>

您的@subjects对象未定义。您需要在控制器操作中为此页面设置一些设置该变量内容的内容,例如:

@subjects = Subject.all 

查看options_from_collection_for_select的源代码 - 它首先做的是对传递给它的集合进行地图调用(在你的情况下是@subjects)。


1
投票
<%= collection_select(:book,:subject_id,Subject.all,:id,:name) %>

0
投票

对于看这篇文章的人而言,答案并不适合他们;

我试图将“Departments”表中的“Locations”表链接起来。

以下代码对我有用,从原始答案中汲取灵感;

# In app/views/departments/_form.html.erb:
...
    <p>
        <%= f.label :location %><br>
        <%= f.collection_select(:location,@locations,:id,:address) %>
    </p>    
...

# In app/controllers/departments_controller.rb;

  def new
    @locations = Location.all
  ....
  end
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