Using Mox to mock DateTime used during init

I like this idea by @benwilson512

Yeah, that’s probably going to work better than all the mocking. Will refactor it and see how it looks. Thanks.

I tend to not unit tests processes started by the supervision tree, but setup separate instances I test. This also allows those tests to be able to run async.

@LostKobraki That makes sense. You mean calling GenServer.start from inside the tests, then exercising that instance, right?

There’s start_supervised in ex_unit, which will handle proper cleanup automatically.

I think what you want to do is run Mox.stub_with immediately after Application.ensure_all_started(:mox) in test_helpers.exs. IIRC the Application tree is started as part of ExUnit.start

If this doesn’t work, as kobrakai suggests, you can do a thing where your server isn’t started in the Application tree, gated on Mix.env == :test (just remember to make sure it’s a compile time gate and not a runtime instruction); and you can manually start the server in test_helpers.

This is usually fine, but you are then never really testing the presence of the default argument. That may be fine for you or not.

I personally wrap DateTime in a helper, so I can make methods like Date.greater_than?

BUT regardless if you mock the actual datetime module and use an expect in the test it will work

def init() do
  ..
  MyApp.DateTimeUtils.utc_now()
 ...
end

Then set up mox as usual:

# test.exs
config(:my_app, date_time_module: DateTimeMock)

# somewhere:
Mox.defmock(DateTimeMock, for: DateTime.Behaviour)

# the behaviour
defmodule DateTime.Behaviour do
    @callback utc_now :: DateTime.t()
end

# the Utils Module
defmodule MyApp.DateTimeUtils do
  @behaviour DateTime.Behaviour

  @impl true
  def utc_now() do
     date_time().utc_now()
  end

  defp date_time(), do: Application.get_env(:my_app, :date_time_module, DateTime)
end

Thanks @Adzz. Worked for us.

Our resulting code looks very similar . Posting here for inspiration.

# lib/my_app/date_time.ex
defmodule MyApp.DateTime do
  @callback now :: DateTime.t()

  def now() do
    Timex.now()
  end

  def instance() do
    Application.get_env(:my_app, :date_time_module, MyApp.DateTime)
  end
end

# test/support/date_time_test_helper.ex
Mox.defmock(MyApp.DateTimeMock, for: MyApp.DateTime)

defmodule MyApp.DateTimeTestHelper do
  def setup do
    Application.put_env(
      :my_app,
      :date_time_module,
      MyApp.DateTimeMock
    )

    :ok
  end

  def stub_current_time(now) do
    Mox.stub(MyApp.DateTimeMock, :now, fn -> now end)
  end
end

# test file
  setup do
    MyApp.DateTimeTestHelper.setup()
  end

  test "if it is midday, does something" do
    MyApp.DateTimeTestHelper.stub_current_time(
      Timex.shift(Timex.now(), day: 1)
    )

    # ... do something
  end