Problem

I have unit tests and integration tests

Both call the same set of functions.

All the integration tests passed. 🎉

All the unit tests failed. 😢

For every unit test , I got the same error :

1

** (UndefinedFunctionError) function HTTPoison.request/5 is undefined (module HTTPoison is not available)

Searching for module not available pointed to solutions that were not applicable. Most suggested including httpoison in mix.exs - which was already taken care of.

I'm grateful for the people on the elixir forum for being helpful and responsive. (You can read the details on elixir forum. Link at the end.)

Root Cause

I am using Mock library in the unit tests. It turns out it changes what is available to the underlying code (as it should).

I was using HTTPoison.post in the unit tests, while one of the lower level function was calling HTTPoison.request

Solution

The fix was as simple as changing HTTPoison.post to HTTPoison.request in the with_mock HTTPoison block.

Reference : See the discussion on Elixir Forum