How to pass additional error details through fallback controller to error view?

Phoenix Forum Questions / Help
#1

Hi there,

I’m trying to figure out how to pass additional information about an error coming from my domain layer which maps to a more generic error, say a “401 Unauthorized”. Put more simply, I’d like the ability to send an optional error string to the fallback controller:

{:error, :unauthorized}
# or
{:error, :unauthorized, "some additional error details"}

So, in my domain layer, I have:

  def login_with_facebook(access_token) do
    case @facebook_api.me("first_name, last_name, email, gender", access_token) do
      {:json, %{"error" => %{"type" => "OAuthException", "message" => message}}} ->
        {:error, :unauthorized, message}
      {:json, %{"error" => %{"message" => message}}} ->
        {:error, :unknown, message}
      {:json, user_data} ->
        # do stuff
    end
  end

Then, in my controller:

  def facebook(conn, %{"access_token" => access_token}) do
    with {:ok, %User{} = user} <- Domain.login_with_facebook(access_token) do
      conn
      |> put_status(:created)
      |> render(UserView, "show.json", user: user)
    end
  end

I’m assuming I’d need to create an additional function in the fallback controller to handle this additional but optional message but it feels a bit awkward:

  def call(conn, {:error, :unauthorized}) do
    conn
    |> put_status(:unauthorized)
    |> render(RevealWeb.ErrorView, :"401")
  end

  def call(conn, {:error, :unauthorized, error}) do
    conn
    |> put_status(:unauthorized)
    |> render(RevealWeb.ErrorView, :"401", error: error)
  end

Curious to know, is there’s a better way to achieve this?

Thanks!

1 Like
#2

This is the answer :slight_smile:

As you showed, you need to expose the error message as part of your return value and then define a clause to handle it in the fallback controller. I wouldn’t consider this awkward and you can compose the fallback controller clauses to accept an error message or not with the same 401 status/template render. Hope that helps!

2 Likes
#3

Sweet, thanks! I wasn’t sure if modifying the conn was possibly preferred or if maybe there was an alternative that I just wasn’t aware of. Being relatively new to Phoenix and Elixir, it’s nice to know the solution is approved of!

1 Like