Rails undefined method `model_name' for NilClass:Class

Question

I have a form that i want to appear at the top of every page so i've included it in the /app/views/layouts/application.html.erb file and i get the error undefined methodmodel_name' for NilClass:Class` when trying to load the page.

Here's the form snippet in application.html.erb

 <%= form_for @user do |f| %>
    <h3>Add new contact</h3>
    <%= f.text_field :first_name %><br />
    <%= f.text_field :last_name %>
    <%= f.text_field :email %>
    <hr />
    <%= f.submit "Add Contact" %>
<% end %>

Here's my /app/controllers/user_controller.rb

class UserController < ApplicationController

    def index

    end

    def new
        @user = User.new
    end

end

I'm thinking that i'm hitting this error because since the form is in the application.html.erb file, i need to somehow specify the path, but then again i'm very new to rails.

Let me know if you need anything else posted.

EDIT Following Ola's suggestion, In my app/views/layouts/application.html.erb file I have something like this:

<div>
<%= yield :add_user %>
</div>

and in my app/views/users/new.html.erb file I have this

<% content_for :add_user do %>
    <%= form_for @user do |f| %>
        <h3>Add new contact</h3>
        First Name<br />
        <%= f.text_field :first_name %><br />
        Last Name<br />
        <%= f.text_field :last_name %><br />
        Email<br />
        <%= f.text_field :email %>
        <hr />
        <%= f.submit "Add Contact" %>
    <% end %>
<% end %>

The form is not rendered because my url is http://localhost:3000/admin/index and so it's looking for the add_user content_for in app/views/admin/index.html.erb

Sorry actually i'm loading this page http://localhost:3000/admin/index — Catfish
Yes, I thought as much, in the example above /users/new should work (kind of), though you've got your view structure all wrong - see my answer below. — Ola Tuvesson
I'm afraid you've got the yield directive upside down too - trust me I know how confusing all this stuff is when you first hit the rails. <%= yield :something %> will render output from the current action, which would vary depending on what page you're on. Great for when you want each controller/action to set the contents of some universal page element (often used to set the document title) but you actually want the complete opposite - to render some component the same on every page. For this you should use <%= render :partial => "thing/partial" %> in your template. — Ola Tuvesson

Ola Tuvesson Ola Tuvesson · Accepted Answer · 2012-06-19T03:07:57

If the form is included for multiple actions (pages) you need to set @user to something or the form_for directive will fail (it won't have anything to create a form for).

A much better approach (and the Rails default) is to have separate views for each action, each including only the elements required by that action, and loaded into your application.html.erb layout with <%= yield %>. So you would have a app/views/users/new.html.erb view which has the form in it. You ~~never~~ very rarely need to define any load paths in Rails, they are all derived from the model, controller and action names - or from your routes.rb. This is a core part of the convention over configuration paradigm which runs so deep in Rails.

Edit: If you do need to have a form for creating a new object on every page (often used for UserSessions for example), you can rewrite your form_for so that it doesn't depend on an object being present:

form_for(User.new)

or if you need to force it to post to the create method

form_for(User.new, :url => { :action => "create" })

You can read more about resource driven form_for in the Ruby On Rails API docs.

Rails undefined method `model_name' for NilClass:Class

5 Answers