User.rb

class User < ApplicationRecord
  include Storext.model
  devise :database_authenticatable, :registerable, :omniauthable,
     :recoverable, :rememberable, :trackable, :validatable
  validates :first_name, :last_name, :email, :experience_level,
        :goal_level, :theoretical_learner_level, presence: true
  mount_uploader :avatar, AvatarUploader

  # Override devise method for Oauth
  def self.new_with_session(params, session)
    if session['devise.user_attributes']
      new(session['devise.user_attributes'].merge(session[:user_attributes])) do |user|
        user.attributes = params
        user.valid?
      end
    else
      super
    end
  end

  def self.from_omniauth(auth)
    where(auth.slice(:provider, :uid).to_hash).first_or_create do |user|
      OauthUserGenerator.new(user: user, auth: auth).generate
    end
  end

  # If sign in through Oauth, don't require password
  def password_required?
    super && provider.blank?
  end

  # Don't require update with password if Oauth
  def update_with_password(params, *options)
    if encrypted_password.blank?
      update_attributes(params, *options)
    else
      super
    end
  end
end

oauth_user_generator.rb

class OauthUserGenerator
  def initialize(user:, auth:)
    @user = user
    @auth = auth
  end

  def generate
    @user.provider = @auth.provider
    @user.uid = @auth.uid
    @user.email = @auth.info.email
    @user.password = Devise.friendly_token[0, 20]
    @user.first_name = @auth.info.name.split[0]
    @user.last_name = @auth.info.name.split[1]
    @user.remote_avatar_url = @auth.info.image
  end
end

omniauth_callbacks_controller.rb

class OmniauthCallbacksController < Devise::OmniauthCallbacksController
  def omniauth_providers
    process_oauth(request.env['omniauth.auth'].merge(session.fetch(:user_attributes, {})))
  end

  alias facebook omniauth_providers
  alias github omniauth_providers

  private

  def process_oauth(omniauth_params)
    user = User.from_omniauth(omniauth_params)
    if user.persisted?
      flash.notice = 'Signed in!'
      sign_in_and_redirect user
    else
      session['devise.user_attributes'] = user.attributes
      redirect_to new_user_email_registration_path
    end
  end
end