Ruby-on-rails – undefined method `each’ for nil:NilClass… why

ruby-on-railsundefined

rails guide example, click the button save post, console show this message:

Started POST "/posts" for 127001 at 2013-12-25 22:42:04 +0800
Processing by PostsController#create as HTML Parameters:
{"utf8"=>"✓", "authenticity_token"=>"CLalUww3gqnSlED0AWdou6P/U2qya
vPqDiBANQOuYgA=", "post"=>{"title"=>"11", "text"=>"22"},
"commit"=>"Save Post"} (0.0ms) begin transaction (0.0ms)
rollback transaction Redirected to http:// 127001:3000/posts
Completed 302 Found in 16ms (ActiveRecord: 0.0ms)

Started GET "/posts" for 127001 at 2013-12-25 22:42:04 +0800
Processing by PostsController#index as HTML Rendered
posts/index.html.erb within layouts/application (15.6ms) Completed 500
Internal Server Error in 31ms

ActionView::Template::Error (undefined method `each' for
nil:NilClass):
        <th>Text</th>
        </tr>
        <% @posts.each do |post| %>
======================================================

routes is correct, why post is nil? rails 4.0.2 ruby 2.0

Best Answer

In your posts controller, you need to define @posts, which, based on the error you haven't.

# app/controllers/posts_controller.rb
class PostsController < ApplicationController
  def index
    @posts = Post.all
  end
end

As @posts is not defined calling each on it will generate undefined methodeach' for nil:NilClass`.

Related Solutions

Javascript – Detecting an undefined object property

The usual way to check if the value of a property is the special value undefined, is:

if(o.myProperty === undefined) {
  alert("myProperty value is the special value `undefined`");
}

To check if an object does not actually have such a property, and will therefore return undefined by default when you try and access it:

if(!o.hasOwnProperty('myProperty')) {
  alert("myProperty does not exist");
}

To check if the value associated with an identifier is the special value undefined, or if that identifier has not been declared. Note: this method is the only way of referring to an undeclared (note: different from having a value of undefined) identifier without an early error:

if(typeof myVariable === 'undefined') {
  alert('myVariable is either the special value `undefined`, or it has not been declared');
}

In versions of JavaScript prior to ECMAScript 5, the property named "undefined" on the global object was writeable, and therefore a simple check foo === undefined might behave unexpectedly if it had accidentally been redefined. In modern JavaScript, the property is read-only.

However, in modern JavaScript, "undefined" is not a keyword, and so variables inside functions can be named "undefined" and shadow the global property.

If you are worried about this (unlikely) edge case, you can use the void operator to get at the special undefined value itself:

if(myVariable === void 0) {
  alert("myVariable is the special value `undefined`");
}

Javascript – Why is null an object and what’s the difference between null and undefined

(name is undefined)

You: What is name?^(*)
JavaScript: name? What's a name? I don't know what you're talking about. You haven't ever mentioned any name before. Are you seeing some other scripting language on the (client-)side?

name = null;

You: What is name?
JavaScript: I don't know.

In short; undefined is where no notion of the thing exists; it has no type, and it's never been referenced before in that scope; null is where the thing is known to exist, but it's not known what the value is.

One thing to remember is that null is not, conceptually, the same as false or "" or such, even if they equate after type casting, i.e.

name = false;

You: What is name?
JavaScript: Boolean false.

name = '';

You: What is name?
JavaScript: Empty string

^{*: name in this context is meant as a variable which has never been defined. It could be any undefined variable, however, name is a property of just about any HTML form element. It goes way, way back and was instituted well before id. It is useful because ids must be unique but names do not have to be.}

Best Answer

Related Solutions

Javascript – Detecting an undefined object property

Javascript – Why is null an object and what’s the difference between null and undefined

Related Topic