help: validate cursors?

[ceolinrenato]

I'm using an Ecto embedded_schema in order to validate pagination params being passed into the query string of my API.

However I'm wondering how can I check if a cursor is indeed valid and without having to rescue ArgumentError from Paginator.paginate/4

  defp changeset(schema \\ %__MODULE__{}, params) do
    schema
    |> cast(params, @optional)
    |> validate_cursor()
    |> validate_number(:limit, greater_than: 0, message: "must be greater than 0")
  end

  defp validate_cursor(changeset) do
    cursor_before = get_field(changeset, :before)
    cursor_after = get_field(changeset, :after)

    case {cursor_before, cursor_after} do
      {nil, nil} ->
        changeset

      {_cursor_before, nil} ->
        changeset

      {nil, _cursor_after} ->
        changeset

      {_cursor_before, _cursor_after} ->
        changeset
        |> add_error(:before, "use either 'before' or 'after'")
        |> add_error(:after, "use either 'before' or 'after'")
    end
  end