I’m using using mox to help me with my unit testing (for now, I want to mock a function that fetches external API).

my mix.exs :
{:mox, "~> 1.0", only: :test}

My test_helper.exs :

ExUnit.start()

Mox.defmock(MyApp.MockExternalSources, for: MyApp.MyModule.ExternalSources)

My module code (MyApp.MyModule.ExternalSources) :

defmodule MyApp.MyModule.ExternalSources do

    @callback http_get_resource(url :: String.t(), headers :: list()) :: {:ok, map()} | :error
    @callback fetch_one_external_resource(params :: map()) :: map()
  
    require Logger
  
    def fetch_one_external_resource(params) do
      with {:ok, data} <-
        http_get_resource(params["url"], params["headers"]) do
        do_something(data)
      else
        :error -> %{}
      end
    end
  
    def http_get_resource(url, headers) do
      case HTTPoison.get(url, headers) do
        {:ok, %HTTPoison.Response{status_code: 200, body: body}} ->
          {:ok, Poison.decode!(body)}
  
        {:ok, %HTTPoison.Response{status_code: 404}} ->
          :error
  
        {:error, %HTTPoison.Error{reason: _reason}} ->
          :error
      end
    end
  end

And my test module :

defmodule MyApp.MyModule.ExternalSources.Test do
  use ExUnit.Case, async: true
  import Mox

  setup :verify_on_exit!

  test "fetch and process external data" do

    MyApp.MockExternalSources
    |> expect(:http_get_resource, fn _url, _headers ->
      {:ok, %{status_code: 200, body: %{key1: "value1", key2: "value2"}}}
    end)

    params = %{
      "url" => "xxx",
      "headers" => "yyy"
    }

    assert MyApp.MyModule.ExternalSources.fetch_one_external_resource(params) ==
             %{"new_key1" => "value1", "new_key2" => "value2"}
  end
end

But I always get a failed assertion and the behaviour is that the right value (MyApp.MyModule.ExternalSources.fetch_one_external_resource(params)) is actually calling the external API and getting the real values ignoring my static data in the mock (%{key1: "value1", key2: "value2"})

Any help please !