defmodule MyApp.Book.Update do
  use ExOperation, params: %{
    id!: :integer,
    title!: :string,
    author_id: :integer
  }

  def validate_params(changeset) do
    changeset
    |> Ecto.Changeset.validate_length(:title, min: 5)
  end

  def call(operation) do
    operation
    |> find(:book, schema: MyApp.Book, preload: [:author])
    |> find(:author, schema: MyApp.Author, id_path: [:author_id], optional: true)
    |> step(:result, &do_update(operation.context, &1))
    |> after_commit(:notification, &send_notifcation(&1.result))
  end

  defp do_update(context, txn) do
    txn.book
    |> Ecto.Changeset.cast(params, [:title])
    |> Ecto.Changeset.put_assoc(:author, txn.author)
    |> Ecto.Changeset.put_assoc(:updated_by, context.current_user)
    |> MyApp.Repo.update()
  end

  defp send_notification(updated_book) do
    # …
  end
end

context = %{current_user: current_user}

with {:ok, %{result: book}} <- MyApp.Book.Update |> ExOperation.run(context, params) do
  # …
end

def deps do
  [
    {:ex_operation, "~> 0.2.0"}
  ]
end

config :ex_operation,
  repo: MyApp.Repo