Absinthe KV Dataloader Problems

I’ve got a KV Dataloader configured as:

defmodule FoobarWeb.Graphql.CircleMock do
  require Logger

  @circles {
    %{id: "foo", name: "Foo"},
    %{id: "bar", name: "Bar"}
  }

  def data() do
    Dataloader.KV.new(&fetch/2)
  end

  def fetch(batch_key, ids) do
    Logger.debug("fetch batch_key=#{inspect(batch_key)} ids=#{inspect(ids)}")
    for id <- ids do
      Logger.debug("id=#{inspect(id)}")
    end
    %{%{} => nil}
  end

  defp find_circle(id) do
    Logger.debug("find circle #{id}")
    @circles
    |> Enum.find(fn(t) -> t |> Map.get(:id) == id end)
  end
end

and a schema defined as:

defmodule FoobarWeb.Graphql.Schema do
  alias FoobarWeb.Graphql.CircleMock
  use Absinthe.Schema
  import Absinthe.Resolution.Helpers, only: [dataloader: 1]
  require Logger

  object :circle do
    field :id, :id
    field :name, :string
  end

  object :circle_queries do
    field :circle_list, list_of(:circle), resolve: dataloader(CircleMock)
  end

  query do
    import_fields(:circle_queries)
  end

  def context(ctx) do
    loader =
      Dataloader.new
      |> Dataloader.add_source(CircleMock, CircleMock.data())

    Map.put(ctx, :loader, loader)
  end

  def middleware(middleware, _field, %Absinthe.Type.Object{identifier: identifier})
    when identifier in [:query, :subscription, :mutation] do
      [FoobarWeb.Graphql.Auth | middleware]
  end

  def middleware(middleware, _field, _object) do
    middleware
  end

  def plugins do
    [Absinthe.Middleware.Dataloader] ++ Absinthe.Plugin.defaults()
  end
end

When I issue a query I see this printed:
23:23:43.682 [debug] fetch batch_key={:circle_list, %{}} ids=MapSet.new([%{}]) { }

Basic on the docs shouldn’t the batch_key be just :circle_list? Have I done something stupid somewhere?

I based this on the docs available here: Dataloader — absinthe v1.11.0