Created: 2008-10-06 11:43
Updated: 2019-02-06 11:24
License: mit


Machinist 2

Fixtures aren't fun. Machinist is.

If you want Machinist 1, go here.


Note: Machinist isn't under active development. See the Status section below for more info.

Machinist makes it easy to create objects for use in tests. It generates data for the attributes you don't care about, and constructs any necessary associated objects, leaving you to specify only the fields you care about in your test. For example:

describe Comment, "without_spam scope" do
  it "doesn't include spam" do
    # This will make a Comment, a Post, and a User (the author of the
    # Post), generate values for all their attributes, and save them:
    spam = Comment.make!(:spam => true)

    Comment.without_spam.should_not include(spam)

You tell Machinist how to do this with blueprints:

require 'machinist/active_record'

User.blueprint do
  username { "user#{sn}" }  # Each user gets a unique serial number.

Post.blueprint do
  title  { "Post #{sn}" }
  body   { "Lorem ipsum..." }

Comment.blueprint do
  email { "commenter#{sn}" }
  body  { "Lorem ipsum..." }


Upgrading from Machinist 1

See the wiki.

Rails 3

In your app's Gemfile, in the group :test section, add:

gem 'machinist', '>= 2.0.0.beta2'

Then run:

rails generate machinist:install

If you want Machinist to automatically add a blueprint to your blueprints file whenever you generate a model, add the following to your config/application.rb inside the Application class:

config.generators do |g|
  g.fixture_replacement :machinist

Rails 2

See the wiki.



A blueprint describes how to generate an object. The blueprint takes care of providing attributes that your test doesn't care about, leaving you to focus on just the attributes that are important for the test.

A simple blueprint might look like this:

Post.blueprint do
  title  { "A Post" }
  body   { "Lorem ipsum..." }

You can then construct a Post from this blueprint with:


When you call make!, Machinist calls, then runs through the attributes in your blueprint, calling the block for each attribute to generate a value. It then saves and reloads the Post. (It throws an exception if the Post can't be saved.)

You can override values defined in the blueprint by passing a hash to make:

Post.make!(:title => "A Specific Title")

If you want to generate an object without saving it to the database, replace make! with make.

Unique Attributes

For attributes that need to be unique, you can call the sn method from within the attribute block to get a unique serial number for the object.

User.blueprint do
  username { "user-#{sn}" }


If your object needs associated objects, you can generate them like this:

Comment.blueprint do
  post { Post.make }

Calling Comment.make! will construct a Comment and its associated Post, and save both.

Machinist is smart enough to look at the association and work out what sort of object it needs to create, so you can shorten the above blueprint to:

Comment.blueprint do

If you want to override the value for post when constructing the comment, you can do this:

post = Post.make(:title => "A particular title)
comment = Comment.make(:post => post)

For has_many and has_and_belongs_to_many associations, you can create multiple associated objects like this:

Post.blueprint do
  comments(3)  # Makes 3 comments.

Named Blueprints

Named blueprints let you define variations on an object. For example, suppose some of your Users are administrators:

User.blueprint do
  name  { "User #{sn}" }
  email { "user-#{sn}" }

User.blueprint(:admin) do
  name  { "Admin User #{sn}" }
  admin { true }



will use the :admin blueprint.

Named blueprints call the default blueprint to set any attributes not specifically provided, so in this example the email attribute will still be generated even for an admin user.

You must define a default blueprint for any class that has a named blueprint, even if the default blueprint is empty.

Blueprints on Plain Old Ruby Objects

Machinist also works with plain old Ruby objects. Let's say you have a class like:

class Post
  extend Machinist::Machinable

  attr_accessor :title
  attr_accessor :body

You can blueprint the Post class just like anything else:

Post.blueprint do
  title { "A title!" }
  body  { "A body!" }

And Post.make will construct a new Post.

Other Tricks

You can refer to already assigned attributes when constructing a new attribute:

Post.blueprint do
  author { "Author #{sn}" }
  body   { "Post by #{}" }

More Details

Read the code! No, really. I wrote this code to be read.

Check out the specs, starting with the spec for Machinable.


I've tested this with:

Ruby versions: 1.8.7, 1.9.2, 1.9.3, 2.0.0 Rails versions: 2.3, 3.0, 3.2

It may well be happy with other versions too, but I'm not promising anything. Compatibility patches are welcome.


The Machinist specs and source code were written to be read, and I'm pretty happy with them. Don't be afraid to have a look under the hood!

If you want to submit a patch:

  • Fork the project.
  • Make your feature addition or bug fix.
  • Add tests for it. This is important so I don't break it in a future version unintentionally.
  • Commit, do not mess with rakefile, version, or history. (if you want to have your own version, that is fine but bump version in a commit by itself I can ignore when I pull)
  • Send me a pull request. Bonus points for topic branches.


In active use in a number of large Rails 2 and 3 apps.

Development is sporadic at best, as I find myself with less and less need for factories in tests. See Bo Jeanes' excellent article on the topic.

If anybody wants to take over maintenance, let me know.


Machinist is maintained by Pete Yandell (, @notahat)

Other contributors include:

Marcos Arias, Jack Dempsey, Jeremy Durham, Clinton Forbes, Perryn Fowler, Niels Ganser, Jeremy Grant, Jon Guymon, James Healy, Ben Hoskings, Evan David Light, Chris Lloyd, Adam Meehan, Kyle Neath, Lawrence Pit, Xavier Shay, T.J. Sheehy, Roland Swingler, Gareth Townsend, Matt Wastrodowski, Ian White

Thanks to Thoughtbot's Factory Girl. Machinist was written because I loved the idea behind Factory Girl, but I thought the philosophy wasn't quite right, and I hated the syntax.

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