auth_controller.ex

defmodule DokkitoWeb.AuthController do
  use DokkitoWeb, :controller

  # ...
  
  def callback(%{assigns: %{ueberauth_auth: auth}} = conn, _params) do
    case Auth.find_or_create(auth) do
      {:ok, user} ->
        conn
        |> put_flash(:info, "Successfully authenticated.")
        |> Guardian.Plug.sign_in(user, key: :user)
        |> redirect(to: dashboard_home_path(conn, :index))
      {:error, reason} ->
        conn
        |> put_flash(:error, reason)
        |> redirect(to: "/")
    end
  end
end

guardian_pipeline.ex

defmodule DokkitoWeb.Plug.GuardianPipeline do
  use Guardian.Plug.Pipeline, otp_app: :dokkito,
                              module: DokkitoWeb.Guardian,
                              error_handler: DokkitoWeb.Plug.AuthErrorHandler,
                              key: :user

  plug Guardian.Plug.VerifySession
  plug Guardian.Plug.EnsureAuthenticated
  plug Guardian.Plug.LoadResource
end