How can i refresh token using omniauth gem? - Rails 4.2

So far i have managed to login users with google plus using the omniauth-google-oauth2 gem.

As far as i am concerned you get an access token that expires at some time. How can i refresh that token from the google API lets say once a week if the user logins with google plus on my website in a week from now?

below is the code i have in my sessioncontroller.rb

def google_oauth2
    auth_hash = request.env['omniauth.auth']

    @identity = Identity.find_by_provider_and_uid(auth_hash["provider"], auth_hash["uid"])
    if @identity
      log_in @identity.user
      flash[:success] = "Welcome, #{@identity.user.name}!"
    else
      random_password = SecureRandom.urlsafe_base64(n=6)
      #@user = create_with_omniauth(auth_hash)
      @user = User.new(
        name:auth_hash['info']['name'],
        email:auth_hash['info']['email'],
        password:random_password,
        password_confirmation:random_password,
        activated:true
      )
      @user.identities.build(
        uid: auth_hash['uid'],
        provider: auth_hash['provider'],
        image_url: auth_hash['info']['image'],
        oauth_token: auth_hash['credentials']['token'],
        oauth_refresh_token: auth_hash['credentials']['refresh_token'],
        oauth_expires_at: auth_hash['credentials']['expires_at']
      )
      if @user.save!
        log_in @user
        flash[:success] = "Hello, #{@user.name}! An account has been created for you!"
      else
        flash[:warning] = "There was an error while trying to authenticate you..."
      end
    end
    redirect_to root_path
  end

some attributes are saved in user table and some in Identity table with a has_many identities, belongs to user model because in the future i want to add multiple providers.