devise.rb

Devise.setup do |config|
  config.mailer_sender = "[email protected]"
  require 'devise/orm/active_record'
  config.case_insensitive_keys = [ :email ]
  config.strip_whitespace_keys = [ :email ]
  config.skip_session_storage = [:http_auth]
  config.stretches = Rails.env.test? ? 1 : 10
  config.reconfirmable = true
  config.password_length = 8..128
  config.reset_password_within = 6.hours
  config.sign_out_via = :delete

  config.warden do |manager|
    manager.intercept_401 = false
    manager.default_strategies(:scope => :user).unshift :remote
  end
end
Devise.add_module :remote_authenticatable, :controller => :sessions, :route => { :session => :routes }

model.rb

require 'devise/strategies/remote_authenticatable'
#from here: https://gist.github.com/madtrick/3916999
module Devise
  module Models
    module RemoteAuthenticatable
      extend ActiveSupport::Concern
      #
      # Here you do the request to the external webservice
      #
      # If the authentication is successful you should return
      # a resource instance
      #
      # If the authentication fails you should return false
      #
      def remote_authentication(authentication_hash)
        binding.pry
        #construct a user instance from authentication_hash (Her::Resource)
        #sign_in = SignIn.new(authentication_hash)
        #sign_in.save() #trigger post
      end

      module ClassMethods
        ####################################
        # Overriden methods from Devise::Models::Authenticatable
        ####################################

        #
        # This method is called from:
        # Warden::SessionSerializer in devise
        #
        # It takes as many params as elements had the array
        # returned in serialize_into_session
        #
        # Recreates a resource from session data
        #
        def serialize_from_session(id, email)
          resource = self.new
          resource.email = email
          resource.id = id
          resource
        end

        #
        # Here you have to return an array with the data of your resource
        # that you want to serialize into the session
        #
        # You might want to include some authentication data
        #
        def serialize_into_session(record)
          [record.id, record.email]
        end
      end
    end
  end
end

strategy.rb

#from here: https://gist.github.com/madtrick/3917079
require 'devise/strategies/authenticatable'

module Devise
  module Strategies
    class RemoteAuthenticatable < Authenticatable
      def valid?
        binding.pry
        #params[:user] && params[:user][:email] && params[:user][:password]
        true
      end
      #
      # For an example check : https://github.com/plataformatec/devise/blob/master/lib/devise/strategies/database_authenticatable.rb
      #
      # Method called by warden to authenticate a resource.
      #
      def authenticate!
        binding.pry #NEVER GETS HERE ?!
        #
        # authentication_hash doesn't include the password
        #
        auth_params = authentication_hash
        auth_params[:password] = password

        #
        # mapping.to is a wrapper over the resource model
        #
        resource = mapping.to.new

        return fail! unless resource

        # remote_authentication method is defined in Devise::Models::RemoteAuthenticatable
        #
        # validate is a method defined in Devise::Strategies::Authenticatable. It takes
        #a block which must return a boolean value.
        #
        # If the block returns true the resource will be loged in
        # If the block returns false the authentication will fail!
        #
        if validate(resource){ resource.remote_authentication(auth_params) }
          success!(resource)
        end
      end
    end
  end
end

Warden::Strategies.add(:remote_authenticatable, Devise::Strategies::RemoteAuthenticatable)