Using Dataloader with Absinthe to avoid N+1 queries

blog.ex

# my_app/blog.ex
defmodule MyApp.Blog do
  def data(), do: Dataloader.Ecto.new(MyApp.Repo, query: &query/2)

  def query(queryable, _params), do: queryable
end

resolvers_blog.ex

# my_app_web/resolvers/blog.ex
defmodule MyAppWeb.Resolvers.Blog do
  def list_posts(%{}, %Absinthe.Resolution{}) do
    {:ok, Blog.list_posts()}
  end
end

schema.ex

# my_app_web/schema.ex
defmodule MyAppWeb.Schema do
  use Absinthe.Schema
  
  import Absinthe.Resolution.Helpers, only: [dataloader: 2]
  
  alias MyAppWeb.Resolvers
  
  object :author do 
    field :id, non_null(:id)
    field :first_name, :string
  end
  
  object :post do
    field :id, non_null(:id)
    field :title, non_null(:string)
    field :author, non_null(:author),
      resolve: dataloader(MyApp.Blog, :author)
  end

  query do
    field :posts, non_null(list_of(:post)),
      resolve: &Resolvers.Blog.list_posts/2
  end

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

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

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