schema.ex

defmodule ApiWeb.Schema do
  use Absinthe.Schema

  import Absinthe.Resolution.Helpers

  import_types Absinthe.Type.Custom
  
  @desc "A user"
  object :user do
    field :id, non_null(:string)
    field :email, non_null(:string)
  end

  query do
    @desc "Get a user given its id"
    field :user, :user do
      arg :id, non_null(:string)
      resolve dataloader(:user)
    end
  end


  def context(ctx) do
    loader =
      Dataloader.new
      |> Dataloader.add_source(:user, Api.Loaders.Users.data())

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

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

users.ex

defmodule Api.Loaders.Users do
  def data() do
    Dataloader.KV.new(&load/2)
  end

  def load(batch_key, ids) do
    # if we made it this far, we haven't fetched before...
    # this should return a tuple of
    # {id, obj}
    # batch_key is {:user, %{id: "1"}}
    # ids is [%{}]
    %{}
  end
end