Absinthe - Resolvers

Guide to implementing efficient and maintainable resolvers in Absinthe.

Key Concepts

Basic Resolvers

defmodule MyApp.Resolvers.User do alias MyApp.Accounts

def list_users(_parent, args, _resolution) do {:ok, Accounts.list_users(args)} end

def get_user(_parent, %{id: id}, _resolution) do case Accounts.get_user(id) do nil -> {:error, "User not found"} user -> {:ok, user} end end

def create_user(_parent, %{input: input}, _resolution) do case Accounts.create_user(input) do {:ok, user} -> {:ok, user} {:error, changeset} -> {:error, changeset} end end end

Resolution Context

def current_user(_parent, _args, %{context: %{current_user: user}}) do {:ok, user} end

def current_user(_parent, _args, _resolution) do {:error, "Not authenticated"} end

Dataloader Integration

defmodule MyApp.Schema do use Absinthe.Schema

def context(ctx) do loader = Dataloader.new() |> Dataloader.add_source(MyApp.Repo, Dataloader.Ecto.new(MyApp.Repo))

Map.put(ctx, :loader, loader)

end

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

In type definitions

object :user do field :posts, list_of(:post) do resolve dataloader(MyApp.Repo) end end

Custom Dataloader Source

defmodule MyApp.Loaders.User do def data() do Dataloader.KV.new(&fetch/2) end

defp fetch(:posts_count, user_ids) do counts = MyApp.Posts.count_by_user_ids(user_ids)

Map.new(user_ids, fn id ->
  {id, Map.get(counts, id, 0)}
end)

end end

Best Practices

• Use Dataloader - Prevents N+1 queries

• Keep resolvers thin - Delegate to context modules

• Handle errors gracefully - Return meaningful error messages

• Use middleware - For cross-cutting concerns like auth

• Batch related queries - Use dataloader batching

Middleware

defmodule MyApp.Middleware.Auth do @behaviour Absinthe.Middleware

def call(resolution, _config) do case resolution.context do %{current_user: %{}} -> resolution _ -> resolution |> Absinthe.Resolution.put_result({:error, "Unauthorized"}) end end end

Apply to fields

field :admin_data, :string do middleware MyApp.Middleware.Auth resolve &MyApp.Resolvers.Admin.get_data/3 end

Error Handling

defmodule MyApp.Resolvers.Helpers do def handle_changeset_errors(%Ecto.Changeset{} = changeset) do errors = Ecto.Changeset.traverse_errors(changeset, fn {msg, opts} -> Enum.reduce(opts, msg, fn {key, value}, acc -> String.replace(acc, "%{#{key}}", to_string(value)) end) end)

{:error, errors}

end end

Anti-Patterns

• Avoid business logic in resolvers

• Don't query database directly in resolvers

• Avoid returning raw Ecto changesets

• Don't skip error handling