Using Absinthe Dataloader for top level queries

I’m trying to figure out how to use Dataloader with absinthe for a top level query. I have it working just fine for a field on an object, like so:

defmodule UserSchema do
  use Absinthe.Schema.Notation
  import Absinthe.Resolution.Helpers, only: [dataloader: 1]
  alias Core.Loaders.CardLoader

  object :user do
    field(:cards, list_of(:card), resolve: dataloader(CardLoader))

However, I’d like to also be able to use Dataloader for top level queries like so:

query do
  field(:cards, list_of(:card), resolve: dataloader(CardLoader))

But this fails with this error:

** (Dataloader.GetError)   The given atom - :cards - is not a module.

  This can happen if you intend to pass an Ecto struct in your call to
  `dataloader/4` but pass something other than a struct.

How can I achieve this? I could obviously do it another way, by having the resolver function call Repo.all(Card) but I’d love to be able to do it all with Dataloader. Is that possible?

Usually you need explicit resolvers since most the time a query will receive parameters, from args or from the current logged user, so that you can specify how to fetch the data you want.

I think I can do that same thing in the CardLoader though. The Absinthe Dataloader docs even suggest doing your filtering and sorting in there.

The premise of a GraphQL DataLoader is as an optimization for bundling the visitation of child nodes, not only within the context of a single record’s result tree but, more powerfully, even across similarly typed child nodes in multiple, concurrently- or parallely-executed result trees.

To do this, the pattern typically collects a set of child IDs (associated in some way to the parent object, either through a map-like data structure and/or with closures) that can be executed in batches within predefined batching/execution windows.

I think the problem you’re running into trying to use the data loader for a root query is that it is expecting a parent value, which doesn’t exist because, by definition, there is no parent value at the root (unless you explicitly add it).

I ended up getting this working with the following:

import Absinthe.Resolution.Helpers, only: [on_load: 2]

query do
  field :users, list_of(:user) do
    arg(:ids, non_null(list_of(:id)))
    resolve(fn %{ids: ids}, %{context: %{loader: loader}} ->
      |> Dataloader.load_many(UserLoader, User, ids)
      |> on_load(fn loader ->
        users = Dataloader.get_many(loader, UserLoader, User, ids)
        {:ok, users}