I’ve been using generic actions lately but when they need to return {:error, whatever} things go a bit haywire. Remembering something about Splode from the changelogs I attempted to create custom errors for my application, following the get started with splode guide.

That produces errors that seem to do the right thing, but when a generic action returns {:error, %MyApp.Errors.CustomError{...}} I get a different error.

The following reproduces it:

Mix.install(
  [
    {:ash, "~> 3.0"}
  ],
  consolidate_protocols: false
)

defmodule Accounts.Errors do
  use Splode, error_classes: [
    invalid: Accounts.Errors.Invalid,
    unknown: Accounts.Errors.Unknown,
    custom: Accounts.Errors.Custom
  ],
  unknown_error: Accounts.Errors.Unknown.Unknown
end

defmodule Accounts.Errors.Custom do
  use Splode.ErrorClass, class: :custom
end

defmodule Accounts.Errors.CustomError do
  use Splode.Error, fields: [:thing, :message], class: :custom

  def message(%{thing: thing, message: message}) do
    "Custom error for #{thing}: #{message}"
  end
end

defmodule Accounts.Profile do
  use Ash.Resource,
    domain: Accounts,
    data_layer: Ash.DataLayer.Ets

  actions do
    defaults [:read, :destroy, :create, :update]

    action :fun, :term do
      run fn _, _ ->
        {:error, Accounts.Errors.CustomError.exception(thing: "city hall", message: "really has gone to pot")}
      end
    end
  end
end

defmodule Accounts do
  use Ash.Domain, validate_config_inclusion?: false

  resources do
    resource Accounts.Profile do
      define :fun
    end
  end
end

err = Accounts.Errors.CustomError.exception(thing: "city hall", message: "gone to pot")
IO.inspect(err, label: "error at toplevel")

IO.puts("error from generic action")
Accounts.fun()

# $ elixir ash_error_issue.ex
# error at toplevel: %Accounts.Errors.CustomError{
#   thing: "city hall",
#   message: "gone to pot",
#   splode: nil,
#   bread_crumbs: [],
#   vars: [],
#   path: [],
#   stacktrace: #Splode.Stacktrace<>,
#   class: :custom
# }
# error from generic action
# ** (UndefinedFunctionError) function nil.exception/1 is undefined
#     nil.exception([errors: [%Accounts.Errors.CustomError{thing: "city hall", message: "really has gone to pot", splode: Ash.Error, bread_crumbs: [], vars: [], path: [], stacktrace: #Splode.Stacktrace<>, class: :custom}], splode: Ash.Error])
#     (ash 3.0.9) /home/.../elixir-1.16.2-erts-14.0.2/11d33faefc03b6705b27e772a36495ef/deps/splode/lib/splode.ex:211: Ash.Error.to_class/2
#     (ash 3.0.9) lib/ash/error/error.ex:66: Ash.Error.to_error_class/2
#     (ash 3.0.9) lib/ash.ex:1306: Ash.run_action/2
#     ash_error_issue.ex:64: (file)