Module for signing in users with Facebook. It uses Koala gem to access Graph API. It is meant to be used with devise_token_auth gem. Code parially comes from OmniauthCallbacksController class (from devise_token_auth).

facebook_login_helper.rb

module FacebookLoginHelper

  def login_with_facebook
    return unless fetch_fb_profile

    setup_user_by_fb
    create_token_info
    set_token_on_user

    if User.devise_modules.include?(:confirmable)
      @user.skip_confirmation!
    end

    sign_in(:user, @user, store: false, bypass: false)

    @user.save!
    
    # Code for storing data from params needed for sending push notifications can be injected here:  
    yield @user 

    set_auth_header
    status = @oauth_registration ? :created : :ok
    respond({json: @user.as_json, status: status})
  end


  def fetch_fb_profile
    graph = Koala::Facebook::API.new(fb_access_token)
    begin
      @fb_profile = graph.get_object('me', fields: ['name', 'email'])
    rescue Koala::Facebook::AuthenticationError
      login_failure('Invalid or expired access token.')
      return false
    rescue Faraday::ConnectionFailed
      login_failure('Connection problem.')
      return false
    rescue
      login_failure('Some problem occurred.')
      return false
    end

    unless @fb_profile
      login_failure('No such FB profile.')
      return false
    end

    true
  end


  private
  def login_failure(msg)
    respond({json: {errors: [msg]}, status: 401})
  end


  def set_auth_header
    auth_header = @user.build_auth_header(@token, @client_id)
    response.headers.merge!(auth_header)
  end


  def fb_access_token
    params[:access_token]
  end


  def setup_user_by_fb
    @user = User.where(
        {
            uid: @fb_profile['id'],
            provider: 'facebook'
        }).first_or_initialize

    if @user.new_record?
      @oauth_registration = true
      set_random_password
      set_generated_nickname
    end

    set_email_from_fb

    @user
  end


  def set_email_from_fb
    email = @fb_profile['email']
    @user.email = email if email
  end


  def set_generated_nickname
    fb_name = @fb_profile['name'].gsub(' ', '')
    if User.where(nickname: fb_name).empty?
      @user.nickname = fb_name
    else
      @user.nickname = fb_name + SecureRandom.hex(5)
    end
  end


  def set_random_password
    password = SecureRandom.urlsafe_base64(nil, false)
    @user.password = password
    @user.password_confirmation = password
  end


  def create_token_info
    @client_id = SecureRandom.urlsafe_base64(nil, false)
    @token = SecureRandom.urlsafe_base64(nil, false)
    @expiry = (Time.now + DeviseTokenAuth.token_lifespan).to_i
  end


  def set_token_on_user
    @user.tokens[@client_id] = {
        token: BCrypt::Password.create(@token),
        expiry: @expiry
    }
  end

end