Requiring Sign In in Phoenix
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Last updated 21 September 2016

I talked earlier about creating and using a current_user in Phoenix, but I didn’t explain how to require a current_user for certain controller actions. In Rails I would use a before_filter (or I guess now before_action). I wasn’t sure how to do this in Phoenix, though.

There are two things to figure out:

  1. What is the equivalent of before_action in Phoenix?
  2. How should I share that code in all of my controllers?

Use a plug when you would use a before_filter

You can read the documentation for plugs for more details, but the general idea would be something like:

defmodule App.SomeController do
  use App.Web, :controller

  plug :require_user when action in [:new, :create]

  # actions go here
end

This would run the require_user function before the new and create action, just like you would do with a before_action in Rails.

Put code in web.ex to share it with all controllers

In Rails you can add methods to ApplicationController and, since all your other controllers inherit from it, they will also have the same methods. As such, you define general methods like require_user there so you can use them everywhere.

Elixir is not object oriented so you cannot use inheritance. Instead, there are a series of macros inside the App.Web module that get run for various types of modules. For example, the code inside the controller function gets put into any module where you write use App.Web, :controller. As such, you can add code to that function and all your controllers will “inherit” it.

What should the require_user function look like?

Currently, I have a module like this:

defmodule App.RequireUser do
  def require_user(conn, _) do
    if App.UserSession.current_user?(conn) do
      conn
    else
      conn |>
      Phoenix.Controller.put_flash(:error, "You must be signed in to view that page.") |>
      Phoenix.Controller.redirect(to: "/") |>
      Plug.Conn.halt
    end
  end
end

The general way it works is: if there is a current user, it just returns the conn without changing anything. This means the controller action will happen as normal. If there isn’t a user signed in, I add an error message to the flash, redirect to the home page, and then halt the connection so no further code is run. Depending on your app, you may just want to return a 403 status code and not redirect or do something totally different.

Then, I add import App.RequireUser to the quote do block inside the controller function in App.Web so all the controllers have access to it. I could also just import App.RequireUser in each controller that needs it, but all my controllers need it so it makes sense to do it in a single place for me.

I hope this further helps you dealing with the concept of current_user in your Phoenix applications.

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