beam-community/ex_machina

Create test data for Elixir applications

Source code at GitHub https://hex.pm/packages/ex_machina

{
  "createdAt": "2015-09-14T15:40:28Z",
  "defaultBranch": "main",
  "description": "Create test data for Elixir applications",
  "fullName": "beam-community/ex_machina",
  "homepage": "https://hex.pm/packages/ex_machina",
  "language": "Elixir",
  "name": "ex_machina",
  "pushedAt": "2025-11-24T22:31:06Z",
  "stargazersCount": 2035,
  "topics": [
    "elixir",
    "factories",
    "factory-definitions",
    "testing"
  ],
  "updatedAt": "2025-11-26T16:21:34Z",
  "url": "https://github.com/beam-community/ex_machina"
}

ExMachina

ExMachina makes it easy to create test data and associations. It works great with Ecto, but is configurable to work with any persistence library.

This README follows the main branch, which may not be the currently published version. Here are the docs for the latest published version of ExMachina.

Installation

In mix.exs, add the ExMachina dependency:

def deps do
  [
    {:ex_machina, "~> 2.8.0", only: :test},
  ]
end

Add your factory module inside test/support so that it is only compiled in the test environment.

Next, be sure to start the application in your test/test_helper.exs before ExUnit.start:

{:ok, _} = Application.ensure_all_started(:ex_machina)

Install in just the test environment for non-Phoenix projects

You will follow the same instructions as above, but you will also need to add test/support to your compilation paths (elixirc_paths) if you have not done so already.

In mix.exs, add test/support to your elixirc_paths for just the test env.

def project do
  [app: ...,
   # Add this if it's not already in your project definition.
   elixirc_paths: elixirc_paths(Mix.env)]
end

# This makes sure your factory and any other modules in test/support are compiled
# when in the test environment.
defp elixirc_paths(:test), do: ["lib", "test/support"]
defp elixirc_paths(_), do: ["lib"]

Overview

Check out the docs for more details.

Define factories:

defmodule MyApp.Factory do
  # with Ecto
  use ExMachina.Ecto, repo: MyApp.Repo

  # without Ecto
  use ExMachina

  def user_factory do
    %MyApp.User{
      name: "Jane Smith",
      email: sequence(:email, &"email-#{&1}@example.com"),
      role: sequence(:role, ["admin", "user", "other"]),
    }
  end

  def article_factory do
    title = sequence(:title, &"Use ExMachina! (Part #{&1})")
    # derived attribute
    slug = MyApp.Article.title_to_slug(title)
    %MyApp.Article{
      title: title,
      slug: slug,
      # another way to build derived attributes
      tags: fn article ->
        if String.contains?(article.title, "Silly") do
          ["silly"]
        else
          []
        end
      end,
      # associations are inserted when you call `insert`
      author: build(:user)
    }
  end

  # derived factory
  def featured_article_factory do
    struct!(
      article_factory(),
      %{
        featured: true,
      }
    )
  end

  def comment_factory do
    %MyApp.Comment{
      text: "It's great!",
      article: build(:article),
    }
  end
end

Using factories (check out the docs for more details):

# `attrs` are automatically merged in for all build/insert functions.

# `build*` returns an unsaved comment.
# Associated records defined on the factory are built.
attrs = %{body: "A comment!"} # attrs is optional. Also accepts a keyword list.
build(:comment, attrs)
build_pair(:comment, attrs)
build_list(3, :comment, attrs)

# `insert*` returns an inserted comment. Only works with ExMachina.Ecto
# Associated records defined on the factory are inserted as well.
insert(:comment, attrs)
insert_pair(:comment, attrs)
insert_list(3, :comment, attrs)

# `params_for` returns a plain map without any Ecto specific attributes.
# This is only available when using `ExMachina.Ecto`.
params_for(:comment, attrs)

# `params_with_assocs` is the same as `params_for` but inserts all belongs_to
# associations and sets the foreign keys.
# This is only available when using `ExMachina.Ecto`.
params_with_assocs(:comment, attrs)

# Use `string_params_for` to generate maps with string keys. This can be useful
# for Phoenix controller tests.
string_params_for(:comment, attrs)
string_params_with_assocs(:comment, attrs)

Delayed evaluation of attributes

build/2 is a function call. As such, it gets evaluated immediately. So this code:

insert_pair(:account, user: build(:user))

Is equivalent to this:

user = build(:user)
insert_pair(:account, user: user) # same user for both accounts

Sometimes that presents a problem. Consider the following factory:

def user_factory do
  %{name: "Gandalf", email: sequence(:email, &"gandalf#{&1}@istari.com")}
end

If you want to build a separate user per account, then calling insert_pair(:account, user: build(:user)) will not give you the desired result.

In those cases, you can delay the execution of the factory by passing it as an anonymous function:

insert_pair(:account, user: fn -> build(:user) end)

You can also do that in a factory definition:

def account_factory do
  %{user: fn -> build(:user) end}
end

You can even accept the parent record as an argument to the function:

def account_factory do
  %{user: fn account -> build(:user, vip: account.premium) end}
end

Note that the account passed to the anonymous function is only the struct after it’s built. It’s not an inserted record. Thus, it does not have data that is only accessible after being inserted into the database (e.g. id).

Full control of factory

By default, ExMachina will merge the attributes you pass into build/insert into your factory. But if you want full control of your attributes, you can define your factory as accepting one argument, the attributes being passed into your factory.

def custom_article_factory(attrs) do
  title = Map.get(attrs, :title, "default title")

  article = %Article{
    author: "John Doe",
    title: title
  }

  # merge attributes and evaluate lazy attributes at the end to emulate
  # ExMachina's default behavior
  article
  |> merge_attributes(attrs)
  |> evaluate_lazy_attributes()
end

NOTE that in this case ExMachina will not merge the attributes into your factory, and it will not evaluate lazy attributes. You will have to do this on your own if desired.

Non-map factories

Because you have full control of the factory when defining it with one argument, you can build factories that are neither maps nor structs.

# factory definition
def room_number_factory(attrs) do
  %{floor: floor_number} = attrs
  sequence(:room_number, &"#{floor_number}0#{&1}")
end

# example usage
build(:room_number, floor: 5)
# => "500"

build(:room_number, floor: 5)
# => "501"

NOTE that you cannot use non-map factories with Ecto. So you cannot insert(:room_number).

Usage in a test

# Example of use in Phoenix with a factory that uses ExMachina.Ecto
defmodule MyApp.MyModuleTest do
  use MyApp.ConnCase
  # If using Phoenix, import this inside the using block in MyApp.ConnCase
  import MyApp.Factory

  test "shows comments for an article" do
    conn = conn()
    article = insert(:article)
    comment = insert(:comment, article: article)

    conn = get conn, article_path(conn, :show, article.id)

    assert html_response(conn, 200) =~ article.title
    assert html_response(conn, 200) =~ comment.body
  end
end

Where to put your factories

If you are using ExMachina in all environments:

Start by creating one factory module (such as MyApp.Factory) in lib/my_app/factory.ex and putting all factory definitions in that module.

If you are using ExMachina in only the test environment:

Start by creating one factory module (such as MyApp.Factory) in test/support/factory.ex and putting all factory definitions in that module.

Later on you can easily create different factories by creating a new module in the same directory. This can be helpful if you need to create factories that are used for different repos, your factory module is getting too big, or if you have different ways of saving the record for different types of factories.

Splitting factories into separate files

This example shows how to set up factories for the testing environment. For setting them in all environments, please see the To install in all environments section

Start by creating main factory module in test/support/factory.ex and name it MyApp.Factory. The purpose of the main factory is to allow you to include only a single module in all tests.

defmodule MyApp.Factory do
  use ExMachina.Ecto, repo: MyApp.Repo
  use MyApp.ArticleFactory
end

The main factory includes MyApp.ArticleFactory, so let’s create it next. It might be useful to create a separate directory for factories, like test/factories. Here is how to create a factory:

defmodule MyApp.ArticleFactory do
  defmacro __using__(_opts) do
    quote do
      def article_factory do
        %MyApp.Article{
          title: "My awesome article!",
          body: "Still working on it!"
        }
      end
    end
  end
end

This way you can split your giant factory file into many small files. But what about name conflicts? Use pattern matching to avoid them!

defmodule MyApp.PostFactory do
  defmacro __using__(_opts) do
    quote do
      def post_factory do
        %MyApp.Post{
          body: "Example body"
        }
      end

      def with_comments(%MyApp.Post{} = post) do
        insert_pair(:comment, post: post)
        post
      end
    end
  end
end

# test/factories/video_factory.ex
defmodule MyApp.VideoFactory do
  defmacro __using__(_opts) do
    quote do
      def video_factory do
        %MyApp.Video{
          url: "example_url"
        }
      end

      def with_comments(%MyApp.Video{} = video) do
        insert_pair(:comment, video: video)
        video
      end
    end
  end
end

If you place your factories outside of test/support make sure they will compile by adding that directory to the compilation paths in your mix.exs file. For example for the test/factories files above you would modify your file like so:

...
  defp elixirc_paths(:test), do: ["lib", "test/factories", "test/support"]
...

Ecto

Ecto Associations

ExMachina will automatically save any associations when you call any of the insert functions. This includes belongs_to and anything that is inserted by Ecto when using Repo.insert!, such as has_many, has_one, and embeds. Since we automatically save these records for you, we advise that factory definitions only use build/2 when declaring associations, like so:

def article_factory do
  %Article{
    title: "Use ExMachina!",
    # associations are inserted when you call `insert`
    comments: [build(:comment)],
    author: build(:user),
  }
end

Using insert/2 in factory definitions may lead to performance issues and bugs, as records will be saved unnecessarily.

Passing options to Repo.insert!/2

ExMachina.Ecto uses Repo.insert!/2 to insert records into the database. Sometimes you may want to pass options to deal with multi-tenancy or return some values generated by the database. In those cases, you can use c:ExMachina.Ecto.insert/3:

For example,

# return values from the database
insert(:user, [name: "Jane"], returning: true)

# use a different prefix
insert(:user, [name: "Jane"], prefix: "other_tenant")

Flexible Factories with Pipes

def make_admin(user) do
  %{user | admin: true}
end

def with_article(user) do
  insert(:article, user: user)
  user
end

build(:user) |> make_admin |> insert |> with_article

Using with Phoenix

If you want to keep the factories somewhere other than test/support, change this line in mix.exs:

# Add the folder to the end of the list. In this case we're adding `test/factories`.
defp elixirc_paths(:test), do: ["lib", "test/support", "test/factories"]

Custom Strategies

You can use ExMachina without Ecto, by using just the build functions, or you can define one or more custom strategies to use in your factory. You can also use custom strategies with Ecto. Here’s an example of a strategy for json encoding your factories. See the docs on [ExMachina.Strategy] for more info.

[ExMachina.Strategy] !: https://hexdocs.pm/ex_machina/ExMachina.Strategy.html

defmodule MyApp.JsonEncodeStrategy do
  use ExMachina.Strategy, function_name: :json_encode

  def handle_json_encode(record, _opts) do
    Poison.encode!(record)
  end
end

defmodule MyApp.Factory do
  use ExMachina
  # Using this will add json_encode/2, json_encode_pair/2 and json_encode_list/3
  use MyApp.JsonEncodeStrategy

  def user_factory do
    %User{name: "John"}
  end
end

# Will build and then return a JSON encoded version of the user.
MyApp.Factory.json_encode(:user)

Contributing

Before opening a pull request, please open an issue first.

git clone https://github.com/beam-community/ex_machina.git
cd ex_machina
mix deps.get
mix test

Once you’ve made your additions and mix test passes, go ahead and open a PR!