How to inject module dependencies into Plug routers ?

[Emil Soman]

I've been reading about the various schools of thought wrt testing Elixir code , ie, mocking vs dependency injection vs module swapping using configuration.

I'm personally in favor of injecting dependencies to functions. Now, I am not sure if core libraries like Plug etc have been built around this idea.

Let's say I have a router that looks like this :

```elixir

defmodule MyRouter do

  use Plug.Router

  plug :match

  plug :dispatch

  post "/attack" do

    # missile_launcher.launch_missile -> can the module "missile_launcher" be injected ?

    send_resp(conn, 200, "Missile launched")

  end

  match _ do

    send_resp(conn, 404, "oops")

  end

end

```

How do I inject a module into the router so that I can use it inside the actions (see the comment above).

This would make it easy to test this route using a mock module:

```elixir

defmodule MyRouterTest do

  use ExUnit.Case

  use Plug.Test

  # Can I use a fake MissileLauncher module when initializing the router ?

  @opts MyRouter.init([])

  test "asks Missile module to launch a missile" do

    conn = conn(:post, "/attack")

    conn = MyRouter.call(conn, @opts)

    assert conn.status == 200

    assert conn.resp_body == "Missile launched"

  end

end

```

Does the Plug.Router's init function allow us to pass in dependencies (or any custom options)

that can be accessed inside the route matchers ?

Cheers,

Emil

[José Valim]

You can by using assigns. In your code:

conn.assigns[:missile_launcher] || Default

In your test, after you create the connection but before the call:

assign(conn, :missile_launcher, TestLauncher)

José Valim

www.plataformatec.com.br

Skype: jv.ptec

Founder and Director of R&D

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[Emil S]

Thanks José, this works well! By the way, is this the idiomatic way to test routers ? This is so much cleaner than setting up :meck expectations anyways :)

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[Aaron Jensen]

You can by using assigns. In your code:

conn.assigns[:missile_launcher] || Default

In your test, after you create the connection but before the call:

assign(conn, :missile_launcher, TestLauncher)

We've been using `put_private/3` instead, which puts things in `conn.private`. It felt like a better approach than overriding what `assigns` was used for. Is there anything wrong w/ that approach?

[José Valim]

private is also fine. private was meant for libraries, since this is likely application code, it is fine for you to use assigns as well.

José Valim

www.plataformatec.com.br

Skype: jv.ptec

Founder and Director of R&D

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