OmniAuth

authentication.rb

class Authentication < ActiveRecord::Base
  belongs_to :user

  attr_accessible :provider, :uid, :user_id

  validates :provider, :uid, presence: true
end

new.html.haml

-# app/views/devise/registrations/new.html.haml

%h2.offset2= title t(:sign_up)
= simple_form_for(resource, as: resource_name, url: registration_path(resource_name), html: { class: "form-horizontal" }) do |f|
  = f.error_notification
  .form-inputs
    = f.input :username, required: true, autofocus: true
    = f.input :email, required: true
    - if f.object.password_required?
      = f.input :password, required: true
      = f.input :password_confirmation, required: true
    .control-group
      .controls
        .input
          = f.button :submit, t(:do_sign_up), class: "width220"
    .strikethrough_divider
      .strike
      %p= t(:or)
    .control-group
      .controls
        .input
          = render 'devise/shared/social_links'

= render "devise/shared/links"

omniauth_callbacks_controller.rb

class OmniauthCallbacksController < Devise::OmniauthCallbacksController

  def all
    user = User.from_omniauth(request.env["omniauth.auth"])
    if user.persisted?
      flash.notice = I18n.t("devise.sessions.signed_in")
      sign_in_and_redirect user
    else
      session["devise.user_attributes"] = user.attributes
      session["devise.user_auth_attributes"] = user.authentications.first.attributes
      redirect_to new_user_registration_url
    end
  end

  alias_method :twitter, :all
  alias_method :facebook, :all
  alias_method :vkontakte, :all
  alias_method :google_oauth2, :all
end

user.rb

class User < ActiveRecord::Base
  
  has_many :authentications, dependent: :destroy

  # ...

  def self.from_omniauth(auth)
    authentication = Authentication.where(provider: auth.provider, uid: auth.uid.to_s).first
    if authentication.present?
      user = authentication.user
    else
      user = self.find_or_create_by_email(auth.info.email)
      user.authentications.build provider: auth.provider, uid: auth.uid.to_s
      user.username = auth.info.nickname
      user.firstname = auth.info.first_name
      user.lastname = auth.info.last_name
    end

    user
  end

  def self.new_with_session(params, session)
    if session["devise.user_attributes"]
      new(session["devise.user_attributes"], without_protection: true) do |user|
        user.attributes = params
        user.authentications.build session["devise.user_auth_attributes"].slice("provider", "uid")
        user.valid?
      end
    else
      super
    end
  end

  def password_required?
    super && authentications.blank?
  end

end