How to test that a function is called just once

Questions / Help
1

Perhaps oddly, this is the first time I’ve hit this situation in Elixir: I would like to test that a function has only been called once. My use-case is that I’m writing a wrapper around Ecto’s preload and I want to test that when given a list of structs, the implementation looks something like this:

def my_wrapper(structs, preloads) do
  MyApp.Repo.preload(structs, preloads)
end

and not this:

def my_wrapper(structs, preloads) do
  for struct <- struct do
    MyApp.Repo.preload(struct, preloads)
  end
end

My only idea so far is to create a mock for MyApp.Repo.preload (which is generally considered a no-no) and have it update a count stored in a gen_server every time it’s called. This is certainly a solution, but does anyone have any better ideas? I have looked at the tests for preload itself and did not come away with anything (although perhaps I didn’t look hard enough?)

Asking here before I resort to asking the robots.

2

trace — kernel v10.6.1 ought to do it, just trace it from your test case and make sure you get exactly one message saying it’s been called.

12 Likes
3

Hi, I do this exactly in one of my projects with the trace module! The approach has been a huge help across multiple refactors, to keep performance solid. I love it so much that I wrote a blog post about it.

5 Likes
4

Its can be done in a very ergonomic way with Repatch

First, initialize Repatch in test/test_helper.exs

ExUnit.start()
Repatch.setup()

Second, in test do something like

defmodule WrapperTest do
  use ExUnit.Case, async: true
  use Repatch.ExUnit
  import Repatch, only: [private: 1]
  
  setup do
    Repatch.spy(MyApp.Repo)
  end

  test "my_wrapper/2 preloads once" do
    Wrapper.my_wrapper(structs, preloads)
    assert Repatch.called?(MyApp.Repo.preload(_, _), exactly: :once)
  end
end
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5

I had a very “duh me” moment last night as I was falling asleep when I realized I asked this question while following along in in almost real time with this thread which mentions Repatch for spying.

That’s a great write-up @jstimps, thanks! I really need to start dipping into Erlang libs as these are things I’m aware of but never think to use.

I’m giving @Asd the check as even though it means adding a dependency, I’ve been meaning to give Repatch a try since it was announced and that API is so nice.

Thanks for the answers, all!

4 Likes