Devise "<#User not initialized>" error - ruby-on-rails

Question

I have integrated devise in a pre-existing "User" Model. Here it is how it looks like
User.rb
class User < ActiveRecord::Base
# Include default devise modules. Others available are:
# :confirmable, :lockable, :timeoutable and :omniauthable
devise :database_authenticatable, :registerable,
:recoverable, :rememberable, :trackable, :validatable
include AstroModels::Users::Validation
when i browse any authentication page. i get following error
ActionView::Template::Error (undefined method `[]' for nil:NilClass):
I have debug it using <%resource.inspect%> , i get following error,
<#User not initialized>
on the following line
<%= form_for(resource, :as => resource_name, :url => session_path(resource_name)) do |f| %>
Also, i tried a stack overflow answer by changing
<%= form_for(resource, :as => resource_name, :url => session_path(resource_name)) do |f| %>
to
**<%= form_for(resource_name, :as => resource, :url => session_path(resource_name)) do |f| %>**
But doesn't seem work. Just for info i'm integrating devise in an application that uses rails4.
For just more iinfor i have following in my application helper file.
def resource_name
:user
end
def resource
#resource ||= User.new
end
def devise_mapping
#devise_mapping ||= Devise.mappings[:user]
end
Any suggestion/workaround would be great help thanks.

Answer 1

In my case i was inheriting my custom User Model with ActiveRecord::Base class and by ActiveRecord::Base when we inherit any class with it we can not have our own initialize method in that custom model. So i commented out initialize method in my custom model and the error was gone.
Hope it will help you.

Answer 2

You didn't mention routes.rb or the views you need to create a session, which initializes user. Have you completed those tasks? You should start with the Devise installation generator to add in the code including initializers:
rails generate devise:install
routes.rb has to include devise_for that is set for your application. An example, not necessarily complete for you:
Sampleapp::Application.routes.draw do
root :to => "home#index"
devise_for :users, :controllers => {:registrations => "registrations"}
resources :users
end
You can access the views in the Devise gem, or you can copy them to your own application so they can be customized like this:
rails generate devise:views
This will provide app/views/devise views as well as app/views/layouts needed to login and create a session, among others.