ExZample

A scalable error-friendly factories library for your Elixir apps

Installation

If available in Hex, the package can be installed by adding ex_zample to your list of dependencies in mix.exs:

def deps do
  [
    {:ex_zample, "~> 0.5.0"}
  ]
end

Documentation can be generated with ExDoc and published on HexDocs. Once published, the docs can be found at https://hexdocs.pm/ex_zample.

Quick Start

You can build any struct by passing a struct module:

ExZample.build(User)
# => %MyApp.User{first_name: nil, age: 21}

ExZample will automatically use the default values inside of that struct. If you want to define different values, you need to implement the example/0 callback.

defmodule MyApp.User do
  @behaviour ExZample
  defstruct first_name: nil, age: 21

  @impl true
  def example do
    %__MODULE__{first_name: "Abili De Bob", age: 12}
  end
end

# later
ExZample.build(MyApp.User)
# => %MyApp.User{first_name: "Abili De Bob", age: 12}

If you want to separate your test data from your app code, you can define the factory in a different module:

defmodule MyApp.User do
  defstruct first_name: nil, age: 21
end

defmodule MyApp.Factories.UserFactory do
  @behaviour ExZample

  alias MyApp.User

  @impl true
  def example do
    %User{first_name: "Abili De Bob", age: 12}
  end
end

# later
alias MyApp.Factories.UserFactory
ExZample.build(UserFactory)
# => %MyApp.User{first_name: "Abili De Bob", age: 12}

You can implement the example/1 callback when you want to have full control in how your factories are built:

@impl true
def example(attrs) do
  age = Map.get(attrs, :age, 12) * 2
  first_name = Map.get(attrs, :first_name, "Abilid") <> " De Bob"

  %User{first_name: first_name, age: age}
end

# later
build(:user, first_name: "Alice")
# => %User{first_name: "Alice De Bob", age: 24}

Using aliases

If you want to use a nickname for your factories, you can register aliases. For example, in your test_helper.ex you can call ExZample.config_aliases/1.

# in your test_helper.exs
ExZample.config_aliases(%{user: UserFactory})

# in your app_test.exs
test "activates an user" do
  user = ExZample.build(:user)

  assert :ok == MyApp.active_user(user)
end

Building your factories

You can use the build/2, build_pair/2 and build_list/3 to generate data without side-effects.

ExZample.build(UserFactory, age: 42)
# => %MyApp.User{first_name: "Abili De Bob", age: 42}

ExZample.build_pair(UserFactory, age: 42)
# => {%MyApp.User{first_name: "Abili De Bob", age: 42}, %MyApp.User{first_name: "Abili De Bob", age: 42}}

ExZample.build_list(100, UserFactory, age: 42)
# => [
#      %MyApp.User{first_name: "Abili De Bob", age: 42},
#      %MyApp.User{first_name: "Abili De Bob", age: 42},
#      ...
#   ]

# or using aliases
ExZample.build(:user, age: 42)
# => %MyApp.User{first_name: "Abili De Bob", age: 42}

ExZample.build_pair(:user, age: 42)
# => {%MyApp.User{first_name: "Abili De Bob", age: 42}, %MyApp.User{first_name: "Abili De Bob", age: 42}}

ExZample.build_list(100, :user, age: 42)
# => [
#      %MyApp.User{first_name: "Abili De Bob", age: 42},
#      %MyApp.User{first_name: "Abili De Bob", age: 42},
#      ...
#   ]

When you pass attributes, it will override the default ones defined in your factories.

Sequences

Sequences are global counters that you can user in your tests. When your test suite starts you can create sequences using ExZample.create_sequence. For example, in your test_helper.exs:

# test_helper.exs
:ok = Application.ensure_started(:ex_zample)

ExZample.create_sequence(:order_id)
ExZample.create_sequence(:user_email, &"email_#{&1}@test.test")

Then, in your factories or in your tests you can invoke them using ExZample.sequence/1 this:

defmodule MyApp.Factories.UserFactory do
  alias MyApp.User

  import ExZample

  @impl ExZample
  def example do
    %User{
      email: sequence(:user_email),
      name: "Abili de bob"
    }
  end
end

# or in your tests
test "tracks an order" do
  order_id = ExZample.sequence(:order_id)
  # => 1
end

Sequences are Agent processes, no matter how many processes tries to get the next value, the OTP will guarantee it will always generate a different one for each request.

ExZample and Umbrella apps

If you want to avoid the factories or sequences leaking through different apps in your umbrella, you can add them under a scope.

# in apps/myapp_a/test/test_helper.ex
ExZample.config_aliases(:app_a, %{user: UserFactory})
ExZample.create_sequence(:app_a, :user_email, &"user_#{&1}@test.test")

Then, you can use ExZample.ex_zample/1 to narrow the scope of lookups during in your tests. It fits well with ExUnit @tags and setup/1 callbacks. For example, if you are working in a Phoenix app, you can put general scope enforcement in your test/support/data_case.ex file:

defmodule MyApp.DataCase do
  @moduledoc false

  use ExUnit.CaseTemplate

  using do
    quote do
      # Tells `ExZample` which scope you want to look up for your aliases
      @moduletag ex_zample_scope: :app_a

      import Ecto
      import Ecto.Changeset
      import Ecto.Query
      # import ExZample (optional, import the utility functions like `build/2`, `sequence/1`)

    end
  end

  # Makes `ExZample` narrow the scope of aliases based on the configured tag
  # If you have imported the `ExZample`, you can do: `setup :ex_zample`
  setup &ExZample.ex_zample/1

  setup tags do
    :ok = Sandbox.checkout(Repo)

    unless tags[:async] do
      Sandbox.mode(Repo, {:shared, self()})
    end

    :ok
  end
end

Using the the configuration above and narrowing the scope you can have stronger boundary between your Umbrella apps.

Inspiration

This library was strongly inspired by:

• ExMachina
• FactoryBot

Why not other factories libraries?

Right now you shouldn't change for this one. The other factories libraries has much more features.

However, when your codebase starts to get bigger, it's nice to have a way to split up your factories files in multiple files. Also, it's important when a error happens, it should be explicit and easy to reason about.

Most of other Elixir libraries relies on DLS with macro code, and you need to write macros to split up your factories files. When you get some error in your factory, it's very hard to locate where is the real problem.

This library approach is to rely on vanilla Elixir modules and contracts based on Elixir behaviours. No need to use any macro. The purpose for any macro that can shows up here will be for syntax sugar, not part of the main functionality.

Do I really need a factory library?

Probably not, you can define your own modules to return example structs and insert them with your repo module. However, a factory library can give you some convenient functions for you don't need to reinvent the wheel.