Devise form within a different controller

As Andres says, the form calls helpers which are specified by Devise and so aren't present when you access a Devise form from a non-Devise controller.

To get around this, you need to add the following methods to the helper class of the controller you wish to display the form under. Alternatively, you can just add them to your application helper to make them available anywhere.

  def resource_name
    :user
  end

  def resource
    @resource ||= User.new
  end

  def devise_mapping
    @devise_mapping ||= Devise.mappings[:user]
  end

Source: http://pupeno.com/blog/show-a-devise-log-in-form-in-another-page/

Can try this also...check this question.

Source

<%= form_for("user", :url => user_session_path) do |f| %>
  <%= f.text_field :email %>
  <%= f.password_field :password %>
  <%= f.check_box :remember_me %>
  <%= f.label :remember_me %>
  <%= f.submit 'Sign in' %>
  <%= link_to "Forgot your password?", new_password_path('user') %>
<% end %> 

The form you created works when rendered from a Devise controller because "resource" is defined through Devise. Take a look at the implementation of the Devise SessionsController - from what I understand, you're attempting to replicate the "new" action. The method "build_resource" is probably what you're looking after.

The Warden gem is where the "resource" objects are coming from. If you wish to dig deeper, that'd be the place to look.

To refine on the accepted answer, we use this helper to allow different types of resources:

def resource_name
  @resource_name ||= if admin_controller?
    :admin_user
  else
    :user
  end
end

def resource
  @resource ||= resource_name.to_s.classify.constantize.new
end

def devise_mapping
  @devise_mapping ||= Devise.mappings[resource_name]
end

where admin_controller? is something we have from before in the ApplicationController to handle login redirects:

def admin_controller?
  !devise_controller? and request.path =~ /^\/admin/
end
helper_method :admin_controller?

I was getting the same error undefined local variable or method "resource" you describe from one of my controllers, because my controller base class was missing the following (Rails-API ActionController::API was at fault):

include ActionController::Helpers

Thus the helper methods from Devise could not be resolved in the view.

To make Devise work with Rails-API I needed to include:

class ApplicationController < ActionController::API

  include AbstractController::Rendering
  include AbstractController::Layouts
  include ActionController::MimeResponds
  include AbstractController::Translation
  include ActionController::ImplicitRender
  include ActionController::Helpers