I’m wondering if anyone has any educational (i.e. simplified) examples that demonstrate best practices when it comes to handling bad requests in a Phoenix application. For educational purposes, let’s consider just one case:

Creating a record: how to handle a bad payload
Maybe your router has something like this:

  scope "/api", MyAppWeb do
    resources("/things", ThingController)
  end

And maybe your controller has something like this:

  def create(conn, %{"data" => data}) do
    with {:ok, thing} <- ThingCreate.create(data) do
      conn
      |> put_status(:created)
      |> render("thing.json", thing: thing)
    end
  end

You’ll notice that the route won’t pattern-match if your payload is something like [] (an empty JSON array)…

It’s tempting to write another controller function that would handle the bad data:

  def create(conn, _bad_data) do
    # return an error...

but that’s just smelly

This article looked promising, but it didn’t get into details: https://medium.com/appunite-edu-collection/handling-failures-in-elixir-and-phoenix-12b70c51314b

I think that the correct response for this situation would be a 400 (e.g. 422), but not simply a 404. What is the cleanest way to return an error message with specifics and a proper status code? Is this best handled in the fallback controller? Does anyone have a complete example of this?

Thanks for sharing!