Handling partial failures when exposing external data in a resource

Some context to avoid the XY Problem: I’m trying to expose some state which lives in an external API as a resource field.

Let’s say I have a User resource and I want to retrieve the URL for their Gravatar given their email. My understanding is that I can do this with a calculation

  calculations do
    calculate :gravatar_url, :string, User.Calculations.Gravatar
defmodule MyApp.User.Calculations.Gravatar do
  use Ash.Calculation
  alias MyApp.GravatarAPI

  def calculate(users, _opts, _context) do
    Enum.map(users, fn user ->
      case GravatarAPI.fetch_url(user.email) do
        {:ok, url} -> url
        {:error, _} -> nil

The problem with this solution is that if I retrieve a list of users using, e.g., GraphQL, the current implementation just silently returns nil without letting the user know that, e.g., the API request failed for a specific user (which is something which can potentially be shown in GraphQL errors).

On the other hand, if I return {:error, :something} if anything fails a single failed API request makes the calculation fails for all the users.

Is there a way around this? Am I using the wrong Ash tool?

The other possibility I thought of is using a resource with no data layer instead of a calculation. n that case, if I call both Simple.set_data and Ash.Query.add_error does the whole query fails (so nothing gets exposed from GraphQL) or is the partial data shown alongside the error?

This is a good question :). While I think there is something we could do in the future to make this more seamless, if you have a calculation that can partially fail, the best way to do it is to use an embedded type for the return type.

calculate :gravatar_url, :map, User.Calculations.Gravatar do
  constraints fields:  [
    success: [
      type: :boolean,
      allow_nil?: false
    url: [
      type: :string,
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Thanks, this is indeed an interesting way to go.

I still have to explore how AshGraphql exposes errors, and see if I can expose an error in the retrieval from the API in a similar way

Yeah…honestly this is probably something that we should add as a feature of calculations, so that calculations should be allowed to fail. Fail-able calculations would have something like field: %Ash.CalculationResult{result: "result", errors: []}. Then AshGraphql could use that :slight_smile:

1 Like

The recent refactor of calculation loading would likely make this a relatively simple thing to support, but it would take some time/effort still.