An example of how users and lists could know about each other

user_and_list.ex

defmodule User do
  use Highlander.Object, type: :user

  defobject name: "", email: "", known_lists: []
end

defmodule List do
  use Highlander.Object, type: :list

  defobject title: "", todos: [], allowed_users: []
    
  def handle_update({:insert_todo, list, todo, user}, state) do
    if Enum.any?(list.allowed_users,
      fn id -> id == user.id end) do
        %{todos: [todo | state.todos]}
    end
  end
end