Ruby-on-rails nil:NilClass 的未定义方法`map'，是什么原因造成的？
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/16960294/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
undefined method `map' for nil:NilClass , what causes this?
提问by Hussain Akhtar Wahid 'Ghouri'
Hi i am facing this error , completely new to rails so cant figure out what is causing it
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原因造成的
my newBook.html.erb
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>
my book model : book.rb
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
my subject model : subject.rb
我的主题模型：subject.rb
class Subject < ActiveRecord::Base
attr_accessible :name
has_many :book
end
stack trace is :
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 --
回答by Matt
<%= collection_select(:book,:subject_id,@subjects,:id,:name) %>
Your @subjectsobject is undefined. You need in your controller action for this page something that sets the contents of that variable, for example:
您的@subjects对象未定义。您需要在此页面的控制器操作中设置该变量的内容，例如：
@subjects = Subject.all
See the source for options_from_collection_for_select- first thing it does is a map call on the collection passed to it (in your case @subjects).
请参阅options_from_collection_for_select的来源- 它所做的第一件事是对传递给它的集合进行映射调用（在您的情况下@subjects）。
回答by Deepika
<%= collection_select(:book,:subject_id,Subject.all,:id,:name) %>
回答by TattooedJoey
For people looking at this post and the answer isn't working for them;
对于看这篇文章的人来说，答案对他们不起作用；
I was trying to link the table "Locations" in the table "Departments".
我试图链接“部门”表中的“位置”表。
The following code worked for me, having taking inspiration from the original answer;
以下代码对我有用，从原始答案中获得灵感；
# 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
