Hi! Excuse the brevity; I’m on my phone. This is also untested and based on a quick read of the controller guide (Controllers — Phoenix v1.8.8) and my possibly incorrect memory. (It’s been a while since I’ve worked on my Elixir/Phoenix project!)

That being said, I think the usual way for your lists controller would be something like:

def index(conn, %{"user_id" => user_id}) do
  user_lists = Users.list_user_lists(user_id)
  render(conn, "index.json", user_lists: user_lists)
end

def show(conn, %{"user_id" => user_id, "id" => id}) do
  list = Users.get_user_list(id)
  render(conn, "show.json", list: list)
end

From the guide:

It is a good practice to pattern match against params in the function signature to provide data in a simple package we can pass on to rendering.

In the show action above, it may not be necessary to include user_id (and its binding to ‘user_id’) in the pattern match, depending on whether or not the user id is needed when getting the list/preparing data for the view.

Related: see the output of mix phx.routes as per Routing — Phoenix v1.8.8 to see the param names of your router’s routes to use in the pattern matching.