Why can I pass a struct to the route path helper?

thojanssens1 · January 20, 2020, 1:27pm

Hello

In the router.ex file, I have the following route:

resources "/users", UserController

In a Controller, I can use the route path helper:

redirect(to: Routes.user_path(conn, :show, user))

Why does it work? user is a struct, but the path/url will contain an id.

wolfiton · January 20, 2020, 1:30pm

Have a look here https://hexdocs.pm/phoenix/routing.html#path-helpers

thojanssens1 · January 20, 2020, 1:33pm

In the examples, primitives are passed, e.g.
HelloWeb.Router.Helpers.user_post_path(Endpoint, :show, 42, 17)

So that doesn’t explain why structs can be passed.

wolfiton · January 20, 2020, 1:36pm

They are functions that accepts structs and converts them

github.com

phoenixframework/phoenix/blob/master/lib/phoenix/router/helpers.ex

defmodule Phoenix.Router.Helpers do
  # Module that generates the routing helpers.
  @moduledoc false

  alias Phoenix.Router.Route
  alias Plug.Conn

  @anno (if :erlang.system_info(:otp_release) >= '19' do
    [generated: true, unquote: false]
  else
    [line: -1, unquote: false]
  end)

  @doc """
  Callback invoked by the url generated in each helper module.
  """
  def url(_router, %Conn{private: private}) do
    case private do
      %{phoenix_router_url: %URI{} = uri} -> URI.to_string(uri)
      %{phoenix_router_url: url} when is_binary(url) -> url

This file has been truncated. show original

Scroll down to helpers comment and examine the functions you will see conn structs and endpoints structs

kokolegorille · January 20, 2020, 1:40pm

In particular…

      defp to_param(int) when is_integer(int), do: Integer.to_string(int)
      defp to_param(bin) when is_binary(bin), do: bin
      defp to_param(false), do: "false"
      defp to_param(true), do: "true"
      defp to_param(data), do: Phoenix.Param.to_param(data)

With pattern matching (and guards) it is simple to extract id from struct, or any…

thojanssens1 · January 20, 2020, 1:43pm

Thank you! This is where the code tries to find the value from the struct:

github.com

phoenixframework/phoenix/blob/master/lib/phoenix/param.ex#L84




defimpl Phoenix.Param, for: Map do
  def to_param(map) do
    raise ArgumentError,
      "maps cannot be converted to_param. A struct was expected, got: #{inspect map}"
  end
end


defimpl Phoenix.Param, for: Any do
  defmacro __deriving__(module, struct, options) do
    key = Keyword.get(options, :key, :id)


    unless Map.has_key?(struct, key) do
      raise ArgumentError, "cannot derive Phoenix.Param for struct #{inspect module} " <>
                           "because it does not have key #{inspect key}. Please pass " <>
                           "the :key option when deriving"
    end


    quote do
      defimpl Phoenix.Param, for: unquote(module) do
        def to_param(%{unquote(key) => nil}) do

wolfiton · January 20, 2020, 1:44pm

You are welcome, glad I could help