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.