Don't unshim middleware if they aren't shimmed to begin with

[jeffutter]

Hi there, this PR fixes an issue we were seeing when deployed in an application. I apologize for the lack of tests, but I'm not entirely sure why this error is happening or how to trigger it in tests. I wrote a few tests but they all pass regardless of the change.

Error

We're trying to support "Entity Queries" on an Interface, this is the "receiving end" of the @interfaceObject directive.

Our schema looks roughly like this:

defmodule SchemaWithEntityInterface do
  use Absinthe.Schema
  use Absinthe.Federation.Schema

  query do
    field :named_entity, :named_entity
  end

  interface :named_entity do
    key_fields("name")

    field :name, :string

    resolve_type fn
      %{age: _}, _ -> :person
      %{employee_count: _}, _ -> :business
      _, _ -> nil
    end

    field :_resolve_reference, :named_entity do
      resolve(fn
        _, %{name: "John"}, _ ->
          {:ok, %{__typename: "Person", name: "John", age: 20}}

        _, %{name: "Acme"}, _ ->
          {:ok, %{__typename: "Business", name: "Acme", employee_count: 10}}
      end)
    end
  end

  object :person do
    key_fields("name")

    import_fields(:named_entity, [:name, :_resolve_reference])
    field :age, :integer

    interface :named_entity
  end

  object :business do
    key_fields("name")

    import_fields(:named_entity, [:name, :_resolve_reference])
    field :employee_count, :integer

    interface :named_entity
  end
end

When writing unit tests setup like this in absinthe_federation it works fine. In our application either in tests or deployed I get the following error. I haven't been able to simplify it enough to figure out the difference between running in my application vs. in absinthe_federation unit tests.

#PID<0.78989.0> running MyAppWeb.Endpoint (connection #PID<0.78988.0>, stream id 1) terminated
Server: myapp.com:80 (http)
Request: POST /graphql
** (exit) an exception was raised:
    ** (FunctionClauseError) no function clause matching in Absinthe.Middleware.unshim/2
        (absinthe 1.7.6) lib/absinthe/middleware.ex:279: Absinthe.Middleware.unshim([{Absinthe.Middleware.Telemetry, []}, {{Absinthe.Resolution, :call}, #Function<1.131082921/3 in MyAppWeb.Schema.Types.InterfaceEntity.__absinthe_function__/2>}], LiveWeb.Schema)
        (absinthe_federation 0.5.0) lib/absinthe/federation/schema/entities_field.ex:176: Absinthe.Federation.Schema.EntitiesField.resolve_reference/4
        (absinthe_federation 0.5.0) lib/absinthe/federation/schema/entities_field.ex:140: Absinthe.Federation.Schema.EntitiesField.entity_accumulator/3
        (elixir 1.16.0) lib/enum.ex:1700: Enum."-map/2-lists^map/1-1-"/2
        (absinthe_federation 0.5.0) lib/absinthe/federation/schema/entities_field.ex:86: Absinthe.Federation.Schema.EntitiesField.call/2
        (absinthe 1.7.6) lib/absinthe/phase/document/execution/resolution.ex:234: Absinthe.Phase.Document.Execution.Resolution.reduce_resolution/1
        (absinthe 1.7.6) lib/absinthe/phase/document/execution/resolution.ex:189: Absinthe.Phase.Document.Execution.Resolution.do_resolve_field/3
        (absinthe 1.7.6) lib/absinthe/phase/document/execution/resolution.ex:174: Absinthe.Phase.Document.Execution.Resolution.do_resolve_fields/6

PR

Hopefully, this PR makes sense as-is and is useful. If someone can help me better understand how a middleware gets shimmed and why absinthe_federation has to call the (seemingly private) unshim function. I'd be glad to contribute a more thorough solution with tests.

[kzlsakal]

Hi @jeffutter, thank you for the PR! I used your sample schema in one of my local projects that uses multiple middleware, but I wasn't able to reproduce the error you mentioned. What does the query body look like when you get this error?

@kdawgwilk calling the expert here; do you have any suggestions about how we can go about testing this?

[jeffutter]

Hey. Thanks for the quick response!

The query looks something like this:

query {
  _entities(representations: [{__typename: "NamedEntity", name: "John"}]) {
    ... on NamedEntity {
      name
      ... on Person {
        age
      }
    }
  }
}

I'm pretty stumped as to how our application is different than what I can write in unit tests. My best guess is we're adding a middleware somewhere that somehow messes with the shim stuff.

[kzlsakal]

Hmm I am getting the following response for that with the schema you posted, and no errors:

{
    "data": {
        "_entities": [
            {
                "age": 20,
                "name": "John"
            }
        ]
    }
}

What absinthe/absinthe_federation/elixir versions are you on?

[jeffutter]

@kzlsakal I figured it out! :-D

It happens on Schemas with @schema_provider Absinthe.Schema.PersistentTerm. I've included a failing test that is fixed with the change I made.

Perhaps there's a better way to address it though?

[kzlsakal]

@kzlsakal I figured it out! :-D

It happens on Schemas with @schema_provider Absinthe.Schema.PersistentTerm. I've included a failing test that is fixed with the change I made.

Perhaps there's a better way to address it though?

Brilliant! Thanks for uncovering this issue. I requested a couple of nitty changes, otherwise it looks great!

[jeffutter]

@kzlsakal Should be all tidied up now. Let me know if there's anything else that needs to be changed. Thanks!

[kzlsakal]

Thank you, @jeffutter!