Correctly testing and mocking a GenServer that performs initialisation on start

Questions / Help
,
1

Hi all,

I have a simple GenServer that performs some initialisation on start. That initialisation process involves making some calls to an HTTP API. I have delegated the HTTP communications to a lower level module, which I then mock using Mox. All well and good.

The GenServer is in my supervision tree in a vanilla sort of way so it gets started every time I run mix test. That would be fine except the GenServer starts making calls to my mock before I’ve had a chance to specify what the response should be, and mox raises a Mox.UnexpectedCallError.

So my question is; what’s the most idiomatic way of testing this? Options I’ve considered:

  • Preventing applications from being manually started by using mix test --no-start and then manually starting the pieces I need. This is fine but the auto-start behaviour is working nicely elsewhere so would add more work.
  • Selectively changing which processes are started depending on environment. Again this works fine.

Something tells me there is a better way. Thanks for any help!

2

I would have a default mock in the test.exs config and let this genserver make useless calls. And to actually test the genserver I would call its functions in my test directly, not relying on the actual process that is started in the supervision tree.

3

That’s pretty much as far as I’ve got but since that default mock hasn’t been configured with any expectations yet I get the Mox.UnexpectedCallError situation.

How do you specify your default mock?

4

I don’t know as I have never used a mocking library in elixir yet. If you genserver receives a module then you can implement a NoopHttpClient module maybe.

5

As @lud suggested I’ve ended up creating a “default mock” in the form of a stub.

I followed the instructions laid out here: https://nts.strzibny.name/elixir-mocking-mox/

2 Likes