Phoenix guide, Context: call a plug function in a Router scope

I try to reproduce the “Context” example in the phoenix guide. But instead of defining authenticate_user in the router.ex file, I did it in another module called SessionAuth under the web directory which imports only Plug.Conn with a few aliases MyApp.Router.Helpers, Phoenix.Controller and MyApp.Accounts.

in the router:

scope "/cms", MyAppWeb.CMS, as: :cms do
pipe_through [:browser, &MyApp.SessionAuth.authenticate_user/2]
resources "/pages", PageController

Then I get

= Compilation error in file lib/my_app_web/router.ex ==
** (ArgumentError) argument error
    (plug) lib/plug/builder.ex:186: Plug.Builder.init_plug/1
    (plug) lib/plug/builder.ex:181: anonymous fn/4 in Plug.Builder.compile/3
    (elixir) lib/enum.ex:1811: Enum."-reduce/3-lists^foldl/2-0-"/3
    (plug) lib/plug/builder.ex:181: Plug.Builder.compile/3

when authenticate_user is defined and called from the router, there is no error.


import MyAppWeb.SessionAuth, only: [authenticate_user: 2]
pipe_through [:browser, :authenticate_user]

there is no error

I usually do it using module plugs:

pipeline :auth do
    plug MyApp.Plugs.AuthUser

Then add :auth to your pipeline.

In this case Plugs.AuthUser is a module plug that contains the required init and call functions:

There is possibly a way to it how you’ve described though. I’m not sure what the recommended way would be, but I prefer putting the plug it it’s own module.

I think the issue you’re having is that pipe_through accepts a list of pipelines, so you aren’t able to pass it a function directly.


The module of this authenticate_user function plug already contains a module plug, which add the current_user from the session to the Conn.assigns.

By the way, how should be named this kind of modules?
They aren’t controller because they don’t send any json or http response, they look like middleware of the web directory.

MyAppWeb.SessionAuthService ?
MyAppWeb.SessionAuthMiddleware ?

I think

pipe_through [:browser, &MyApp.SessionAuth.authenticate_user/2]

is the same as

pipe_through [:browser, fn conn, opts -> MyApp.SessionAuth.authenticate_user(conn, opts) end]

and pipe_through macro expects a list of atoms to call something like apply/3 on, not functions.

I usually call these plugs App.Web.Session or App.Web.Auth.


Tks for the explanation.

I ended up with

import MyAppWeb.SessionAuth, only: [authenticate_user: 2] # import plug function
pipe_through [:browser, :authenticate_user]

And if the module MyApp.SessionAuth becomes too big, I will split it into MyApp.Web.Session and MyApp.Web.Auth and use module plugs instead.