Problem mocking a function when doing unit testing

Questions / Help
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1

I’m testing a function that call another function to fetch external data using API calls, here is the main module code :

defmodule MyApp.MyModule.ExternalData do
  require Logger
  
  def fetch_external_data(params) do
    with {:ok, data} <-
      http_get_resource(params["url"], params["headers"]) do
        data
    else
      :error ->
        Logger.info("ERROR executing API call to : #{source} with path : #{url} ")
        %{}
    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 here is my test file :

defmodule MyApp.MyModule.ExternalData.Test do
  use ExUnit.Case

  # Replace `http_get_resource/2` with a mock function to simulate the HTTP request
  def mock_http_get_resource(url, headers) do
    case url do
      # Simulate a successful request with a 200 status code
      "http://valid.com/resource" -> {:ok, %{status_code: 200, body: %{key1: "value1", key2: "value2"}}}
      # Simulate a failed request with a 404 status code
      "http://invalid.com/resource" -> {:ok, %{status_code: 404, body: "Resource not found"}}
      # Simulate a request that times out
      "http://timeout.com/resource" -> {:error, :timeout}
    end
  end

  def http_get_resource(url, headers) do
    mock_http_get_resource(url, headers)
  end

  test "fetch_one_external_resource/1" do
    # Replace the actual `http_get_resource/2` function with the mock function
    alias MyApp.MyModule.ExternalData, as: M
    M.http_get_resource = &http_get_resource/2

    # Test a successful request
    params = %{
      "url": "aaa", 
      "headers": "bbb"
    }
    assert M.fetch_one_external_resource(params) == %{new_key1: "value1", new_key2: "value2"}

    # Test a failed request
    params = %{
      "url": "aaa", 
      "headers": "bbb"
    }
    assert M.fetch_one_external_resource(params) == :error

    # Test a request that times out
    params = %{
      "url": "aaa", 
      "headers": "bbb"
    }
    assert M.fetch_one_external_resource(params) == :error
  end
end

I’m getting this error :

== Compilation error in file test/fetch_data_test.exs ==
** (CompileError) test/fetch_data_test.exs:26: cannot invoke remote function MyApp.MyModule.ExternalData.http_get_resource/0 inside a match
    test/fetch_data_test:23: (module)
    (elixir 1.14.2) lib/kernel/parallel_compiler.ex:455: Kernel.ParallelCompiler.require_file/2
    (elixir 1.14.2) lib/kernel/parallel_compiler.ex:348: anonymous fn/5 in Kernel.ParallelCompiler.spawn_workers/7

Any ideas or fix please, I think the problem is in the line M.http_get_resource = &http_get_resource/2

thanks

2

That’s not how Elixir modules work, you can’t “reassign” functions in them at runtime.

Consider using one of the mocking libraries like mox or meck that does this correctly.

Alternatively, consider refactoring fetch_external_data into separate functions that can be tested without a live HTTP call.

2 Likes
3

Hi @al2o3cr , thank you.

Please see my problem with Mox : Mox is ignoring my Mocked module and calling the real function