How to test a Phoenix Controller with Hammox?

Question

Background

I am trying to create a mock for one of my behaviours. To this end I am using Hammox (a variation of Mox which extends the api).

I have set my application so it gets which module to use from the configuration files, in this case config/test.exs:

config/test.exs

config :my_app, storage: StorageMock

And this is the test file:

test/my_app/application_test.exs

defmodule MyApp.ApplicationTest do
  @moduledoc false

  use ExUnit.Case, async: false
  use MyApp.ConnCase

  import Hammox

  test "client can handle an error response", %{conn: conn} do
    Hammox.defmock(StorageMock, for: MyApp.Storage)

    expect(StorageMock, :get, fn args ->
      assert args == "Chicago"

      # here we decide what the mock returns
      {:ok, %{body: "Some html with weather data"}}
    end)

    get(conn, "~p/api/users/validate")
  end
end

I am trying to test the controller's endpoint,and I am basically mixing the docs of Hammox (https://github.com/msz/hammox) with what i can grasp from a Testing controllers guide (https://hexdocs.pm/phoenix/testing_controllers.html#content).

Problem

Unfortunately, I think my setup is incorrect, as I get this error:

Compiling 1 file (.ex)
** (Mix) Could not start application my_app: exited in: MyApp.Application.start(:normal, [])
    ** (EXIT) an exception was raised:
        ** (ArgumentError) The module StorageMock was given as a child to a supervisor but it does not exist
            (elixir 1.16.0) lib/supervisor.ex:797: Supervisor.init_child/1
            (elixir 1.16.0) lib/enum.ex:1700: Enum."-map/2-lists^map/1-1-"/2
            (elixir 1.16.0) lib/enum.ex:1700: Enum."-map/2-lists^map/1-1-"/2
            (elixir 1.16.0) lib/supervisor.ex:783: Supervisor.init/2
            (elixir 1.16.0) lib/supervisor.ex:707: Supervisor.start_link/2
            (kernel 9.2) application_master.erl:293: :application_master.start_it_old/4

What am I missing here? I assume there is some configuration somewhere to tell the VM that StorageMock is something from the library, but I can't find it.

Answer 1

My solution

Turns out my issue was that I was referencing StorageMock at application startup, namely:

defmodule MyApp.Application do
  @moduledoc false

  use Application

  @impl true
  def start(_type, _args) do
   children = 
    [
      MyAppWeb.Telemetry,
      {Phoenix.PubSub, name: MyApp.PubSub},
      {StorageMock, []},
      MyAppWeb.Endpoint
    ]

    opts = [strategy: :one_for_one, name: MyApp.Supervisor]
    Supervisor.start_link(children, opts)
  end
end

By removing StorageMock from the list of children (when running tests) the error was fixed.

For those of you curious, you can check in which env the application is running, by using Mix.env(). Thus, you can do something like:

  def start(_type, _args) do
    opts = [strategy: :one_for_one, name: MyApp.Supervisor]
    Supervisor.start_link(children(Mix.env(), opts)
  end

  defp children(:test) do
    [
      MyAppWeb.Telemetry,
      {Phoenix.PubSub, name: MyApp.PubSub},
      MyAppWeb.Endpoint
    ]
  end

  defp children(_other_envs) do
    [
      MyAppWeb.Telemetry,
      {Phoenix.PubSub, name: MyApp.PubSub},
      {RealStorage, ["localhost:5221"]},
      MyAppWeb.Endpoint
    ]
  end

This is the solution I ended up going with, as it means you can make use of the config/test.exs to define what modules you want to use. You can even change modules at runtime by using Application.put_env/3 if you are inclined to do that, which can also come in handy when doing some specific types of tests.

Other approaches

Another viable approach can seen in this comment from the community:

test_helper.exs is also loaded after application startup. What works is putting the defmock in a compiled file (not a script), e.g. in test/support/mocks.ex or something like that. See the documentation here: Mox — Mox v1.1.0

Which states that there is a compile time approach to also solving this. Original post: https://elixirforum.com/t/how-to-test-a-phoenix-controller-with-hammox/64191/5?u=fl4m3ph03n1x