Odd bug with Plug.Conn

[António Alvarenga]

I'm having problems using drab in a template with view functions.
The bug is although the page loads fine and I can link an event to the commander, when loading the page it prompts the alert window error without an apparent reason.
This is the error:

[error] Drab Handler failed with the following exception:
** (KeyError) key :phoenix_action not found in: %{phoenix_endpoint: RiskWeb.Endpoint}
    (risk) lib/risk_web/views/evaluation_view.ex:41: RiskWeb.EvaluationView.check_action/3
    (risk) lib/risk_web/templates/evaluation/form.html.drab:101: anonymous fn/2 in RiskWeb.EvaluationView."form.html"/1
    (phoenix_html) lib/phoenix_html/form.ex:291: anonymous fn/2 in Phoenix.HTML.Form.inputs_for/4
    (elixir) lib/enum.ex:1294: Enum."-map/2-lists^map/1-0-"/2
    (phoenix_html) lib/phoenix_html/form.ex:289: Phoenix.HTML.Form.inputs_for/4
    (risk) lib/risk_web/templates/evaluation/form.html.drab:100: anonymous fn/2 in RiskWeb.EvaluationView."form.html"/1
    (phoenix_html) lib/phoenix_html/form.ex:253: Phoenix.HTML.Form.form_for/4
    (risk) lib/risk_web/templates/evaluation/form.html.drab:1: RiskWeb.EvaluationView."form.html"/1
    (phoenix) lib/phoenix/view.ex:332: Phoenix.View.render_to_iodata/3
    (phoenix) lib/phoenix/view.ex:339: Phoenix.View.render_to_string/3
    (drab) lib/drab/live.ex:662: Drab.Live.rerender_template/4
    (drab) lib/drab/live.ex:617: Drab.Live.process_poke/9
    (drab) lib/drab.ex:286: anonymous fn/3 in Drab.handle_callback/3

This is the check_action call in the template:

     ...
     <%= inputs_for f, :activities, fn fa -> %>
         <%= if check_action(@conn, @changeset, fa) do %>
           <tr>
           ...

and the definition in the view:

  def check_action(conn, changeset, fa) do
    if conn.private.phoenix_action === :new and changeset.action === nil do
      fa.data.frequent
    else
      true
    end
  end

The error arises because at a certain point the conn struct looks empty, like this:

%Plug.Conn{
  adapter: {Plug.MissingAdapter, :...},
  assigns: %{},
  before_send: [],
  body_params: %Plug.Conn.Unfetched{aspect: :body_params},
  cookies: %Plug.Conn.Unfetched{aspect: :cookies},
  halted: false,
  host: "www.example.com",
  method: "GET",
  owner: nil,
  params: %Plug.Conn.Unfetched{aspect: :params},
  path_info: [],
  path_params: %{},
  peer: nil,
  port: 0,
  private: %{phoenix_endpoint: RiskWeb.Endpoint},
  query_params: %Plug.Conn.Unfetched{aspect: :query_params},
  query_string: "",
  remote_ip: nil,
  req_cookies: %Plug.Conn.Unfetched{aspect: :cookies},
  req_headers: [],
  request_path: "",
  resp_body: nil,
  resp_cookies: %{},
  resp_headers: [{"cache-control", "max-age=0, private, must-revalidate"}],
  scheme: :http,
  script_name: [],
  secret_key_base: nil,
  state: :unset,
  status: nil
}

despite check_action being called successfully before.
Any ideas on where the problem might be?

[António Alvarenga]

It has nothing to do with using view functions. Accessing Plug.Conn throws the error.

[António Alvarenga]

The problem was I had the callback onload(:page_loaded) in the commander which I had just for testing. Removing it solved the issue

[António Alvarenga]

Reopening this issue because it turns out the error arises when using poke. Any idea on what is causing this?

[António Alvarenga]

Using a partial solves the issue. Can be a bit of a pain if there are several events in the same page.

[Tomek Gryszkiewicz]

Hi @antalvarenga, could you pls be a bit more specific? How can I reproduce it, and how did you solve it using partials?

[António Alvarenga]

Anywhere in the template check something in the conn struct like in my previous comments. Try accessing conn.private.phoenix_action for instance.
Add some event like the uppercase button and the corresponding handler in the commander.
If you poke like this:
poke(socket, text: String.upcase(text))
you should get that error.

Wrapping

<form>
  <input name="text_to_uppercase" value="<%= @text %>">
  <button drab="click:uppercase">Upcase</button>
</form>

in a partial like "text.html.drab" and rendering in the main template like this:
<%= render "text.html", text: @text %>

and poking like this:

defhandler uppercase(socket, sender) do
    text = sender.params["text_to_uppercase"]
    poke(socket, "text.html", text: String.upcase(text))
end

solves the issue.

[António Alvarenga]

I guess each time you poke or do some change on the page it runs all the elixir again, but after the first load the conn struct is no longer available.

[Tomek Gryszkiewicz]

conn.private.phoenix_action

@conn is a special case in Drab, because it is big and contains sensible data. Check the documentation here.

You need to set live_conn_pass_through config to add private to the conn structure.

[António Alvarenga]

Thanks. Yeah I'm not very pleased with that code. I'm accessing private because I have a field coming from the changeset which is pre-populated, and accessing that changes according to the action, like this:

  case conn.private.phoenix_action do
      :edit ->
        changeset.data.activity_name.name

      :update ->
        changeset.data.activity_name.name

      :new ->
        changeset.data.title

      :create ->
        changeset.params["title"]
    end

But it's not a huge deal, right?

[Tomek Gryszkiewicz]

I would do Phoenix.Controller.action_name(conn) instead. But under the hood, this function does the same, accessing private field, so you will need to add it to the live_conn_pass_through config.

Please leave this issue opened, as I think it would be good to add action and controller to the conn by default: it is nothing fragile and not a lot of data.

Thanks for reporting!