Unobtrusive Dependency Injector for Elixir

Why?

Let's say we want to test following function.

def send_welcome_email(user_id) do
  %{email: email} = Repo.get(User, user_id)

  welcome_email(to: email)
  |> Mailer.send()
end

Here's one possible solution to replace Repo.get/2 and Mailer.send/1 with mocks:

def send_welcome_email(user_id, repo \\ Repo, mailer \\ Mailer) do
  %{email: email} = repo.get(User, user_id)

  welcome_email(to: email)
  |> mailer.send()
end

First, I believe that this approach is too obtrusive as it requires modifying the function body to make it testable. Second, with Mailer replaced with mailer, the compiler no longer check the existence of Mailer.send/1.

definject does not require you to modify function arguments or body. It allows injecting different mocks to each function. It also does not limit using :async option as mocks are contained in each test function.

Installation

The package can be installed by adding definject to your list of dependencies in mix.exs:

def deps do
  [{:definject, "~> 1.2"}]
end

By default, definject is replaced with def in all but the test environment. Add the below configuration to enable in other environments.

config :definject, :enable, true

To format definject like def, add following to your .formatter.exs

locals_without_parens: [definject: 1, definject: 2]

Documentation

API documentation is available at https://hexdocs.pm/definject

Usage

use Definject

use Definject transforms def to accept a extra argument deps where dependent functions and modules can be injected.

use Definject

def send_welcome_email(user_id) do
  %{email: email} = Repo.get(User, user_id)

  welcome_email(to: email)
  |> Mailer.send()
end

is expanded into

def send_welcome_email(user_id, deps \\ %{}) do
  %{email: email} =
    Map.get(deps, &Repo.get/2,
      :erlang.make_fun(Map.get(deps, Repo, Repo), :get, 2)
    ).(User, user_id)

  welcome_email(to: email)
  |> Map.get(deps, &Mailer.send/1,
       :erlang.make_fun(Map.get(deps, Mailer, Mailer), :send, 1)
     ).()
end

Note that local function calls like welcome_email(to: email) are not expanded unless it is prepended with __MODULE__.

Now, you can inject mock functions and modules in tests.

test "send_welcome_email" do
  Accounts.send_welcome_email(100, %{
    Repo => MockRepo,
    &Mailer.send/1 => fn %Email{to: "user100@gmail.com", subject: "Welcome"} ->
      Process.send(self(), :email_sent)
    end
  })

  assert_receive :email_sent
end

Function calls raise if the deps includes redundant functions or modules. You can disable this by adding strict: false option.

test "send_welcome_email with strict: false" do
  Accounts.send_welcome_email(100, %{
    &Repo.get/2 => fn User, 100 -> %User{email: "user100@gmail.com"} end,
    &Repo.all/1 => fn _ -> [%User{email: "user100@gmail.com"}] end, # Unused
    strict: false
  })
end

mock

If you don't need pattern matching in mock function, mock/1 can be used to reduce boilerplates.

import Definject

test "send_welcome_email with mock/1" do
  Accounts.send_welcome_email(
    100,
    mock(%{
      Repo => MockRepo,
      &Mailer.send/1 => Process.send(self(), :email_sent)
    })
  )

  assert_receive :email_sent
end

Note that Process.send(self(), :email_sent) is surrounded by fn _ -> end when expanded.

import Definject

import Definject instead of use Definject if you want to manually select functions to inject.

import Definject

definject send_welcome_email(user_id) do
  %{email: email} = Repo.get(User, user_id)

  welcome_email(to: email)
  |> Mailer.send()
end

License

This project is licensed under the MIT License - see the LICENSE file for details