How to use Mox for a module you own?

I cannot seem to get Mox to work for my Accounts module. Here is sort of the code.

# test_helper.exs

ExUnit.start(exclude: [:skip])
Mox.defmock(MyApp.MockAccounts, for: MyApp.Accounts)
# my_app/accounts.exs

defmodule MyApp.Accounts do
  @callback get_role(map()) :: :admin | :user

  def get_role(current_user) do
    # temporary authorization hack
    # yes, i am using my real email (note, real email is not being used here)
    case do
      "" -> :admin
      _ -> :user
# test/integration/admin_area_only_test.exs

defmodule MyApp.Integration.AdminAreaOnlyTest do
  use MyApp.ConnCase, async: true

  import Mox
  setup :verify_on_exit!

  test "admin area only" do
    |> expect(:get_role, fn(_user) -> :admin end)

    # email used here is different because I don't want personal email in tests (yes, a bit ironic, but here we are)
    {:ok, user} = MyApp.Accounts.create_user(%{email: "", password: "123"})

    conn =
      |> put_session(:user_id,
      |> put_session(:current_user, user)


  # assertions
  # it fails because user doesn't have permission

I would rather not have to modify prod.exs, test.exs, and dev.exs because that means I’ll have to use Application.get_env for my Accounts module which is used everywhere. All the Mox examples are used for small libs and external requests like Httpoison but what about an internal module?

Any direction would be greatly appreciated.

Thank you

It’s not really an issue of external vs. internal modules.

You seem to be expecting mox to be able to intercept function calls, which is not possible on the beam. If you call MyApp.Accounts.create_user it’ll call the given module and not MyApp.MockAccounts. There’s no way to intercept calls to MyApp.Accounts and make them be magically resolved by the function on the mock module.

If you want to use mox you need to have means to switch out the real implementation with the mock implemenation in your codebase, so the functions are actually called on the mock module of mox. This can be done using the app env, but there are also other ways to inject the mock module.

Often those means are created automatically for things you expect to switch out anyways like http clients. That might make you feel like Mox is catered specifically for those elements. But the same mechanisms do work for switching out any real implementation with an mocked one. No matter what the code is about.

No matter how you’re going to deal with dependency injection (app env or not) you’re correct in that you don’t want to concern the callers of functions on MyApp.Accounts with the decision which implementation to call. But you can move the code you currently have in MyApp.Accounts to e.g. MyApp.Accounts.Impl and do the decision making within MyApp.Accounts. For every function either forward to MyApp.Accounts.Impl or use a different module, which was somehow injected to be used.


To add to this:

  1. Have a look at It can simplify the boilerplate.
  2. Maybe it’s just my preference, but for behaviour MyApp.Accounts name the implementation - MyApp.AccountsImpl. Then if you use the quick find function in editors, if you type Accounts, you will see both. (some editors like VSCode will find both even in your case, but not all editors). Also I name my mock - MyApp.AccountsMock

I would rather not have to modify prod.exs, test.exs, and dev.exs because that means I’ll have to use Application.get_env for my Accounts module which is used everywhere

Can you go into more detail as to why this is a problem?