-----------------------User.rb-----------------------
def self.find_for_twitter_oauth(auth, signed_in_resource=nil)
  user = User.where(:provider => auth.provider, :uid => auth.uid).first
  unless user
    user = User.create(name:     auth.info.nickname,
                     provider: auth.provider,
                     uid:      auth.uid,
                     email:    User.dummy_email(auth),
                     password: Devise.friendly_token[0,20]
                    )
  end
  user
end

def self.create_unique_string
  SecureRandom.uuid
end

  # Create dummy email for OAuth
def self.dummy_email(auth)
  "#{auth.uid}-#{auth.provider}@example.com"
end

class Users::OmniauthCallbacksController < Devise::OmniauthCallbacksController

  def twitter
  # You need to implement the method below in your model
    @user = User.find_for_twitter_oauth(request.env["omniauth.auth"], current_user)

    if @user.persisted?
      set_flash_message(:notice, :success, :kind => "Twitter") if is_navigational_format?
      sign_in_and_redirect @user, :event => :authentication
    else
      session["devise.twitter_data"] = request.env["omniauth.auth"].except("extra")
      redirect_to new_user_registration_url
    end
  end
end

-------------------routes.rb---------------------------
 devise_for :users, controllers: {
                     sessions: "users/sessions",
                     registrations: "registrations",
                     omniauth_callbacks: "users/omniauth_callbacks"
                   }