Absinthe: Schema, Resolvers & Mutations

Absinthe is the standard GraphQL toolkit for Elixir, most commonly paired with Phoenix -- Phoenix's plug pipeline handles routing to the GraphQL endpoint, and Phoenix Channels power Absinthe's real-time subscriptions.

Defining a Schema

defmodule MyAppWeb.Schema do
  use Absinthe.Schema

  object :user do
    field :id, :id
    field :name, :string
    field :email, :string
  end

  query do
    field :user, :user do
      arg :id, non_null(:id)
      resolve &Resolvers.User.find/3
    end
  end
end

Resolvers

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

# A resolver fetches/computes the value for a specific field --
# returning {:error, ...} surfaces cleanly in GraphQL's errors array

Mutations

mutation do
  field :create_user, :user do
    arg :name, non_null(:string)
    arg :email, non_null(:string)
    resolve &Resolvers.User.create/3
  end
end

# Mutations are for operations with side effects (create/update/delete),
# kept separate from read-only queries by GraphQL convention

Context: Request-Scoped Data

The context is populated per-request -- often with the current authenticated user from an auth plug -- and passed into every resolver, letting resolvers make authorization decisions without relying on global state.