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Add this line to your application's Gemfile:

gem 'contractinator'

And then execute:

$ bundle

Or install it yourself as:

$ gem install contractinator

Then inform RSpec that you'd like to use contractinator by adding something like the following to your spec_helper.rb

require 'contractinator'

RSpec.configure do |config|
  config.include Contractinator::ContractHelpers

  # By default contractinator extends rspec's test doubles.
  # You don't have to use rspec's doubles TODO: explain how
  # to use other mocks.
  config.mock_with :rspec
  # After the suite is done, warn the user about all the
  # unbalanced contracts. 
  config.after(:suite) do
    puts Contractinator::Contract.messages
    puts "#{Contractinator::Contract.fulfilled_set.count} fulfilled contracts"


Creating a Contract

There are several ways to document a provider's behavior. The easiest is to use the stipulate and agree matchers.

In the spec for a consumer, for example a rails controller, you might have

it 'assigns a new entry' do
  stipulate(Entry).must receive(:new).and_return(entry)
  get :new
  expect(response).to be_success
  expect(assigns[:entry]).to eq(entry)

This sets the expectation that will be called, and stubs it out to return entry. Now you should get a warning in your rspec output that looks like this:

unfulfilled contract ' -> entry'
   at spec/controllers/entries_controller_spec.rb:45:in `block (3 levels) in <top (required)>'

The next step is to make sure that contract is fulfilled by something. So we'll switch over to the model spec

describe '.new' do
  it { agree(Entry, :new).will be_a(Entry) }

This calls new on Entry and asserts that it is_a Entry, and fulfills a contract of the form -> entry. Since this matches the one from above, your spec output won't show the unmatched on anymore, but will increment the fulfilled contracts message.

Less straight-forward contracts

Not every contract in an application is so easy to specify. For example, a view spec which assigns a local variable has an agreement with a controller to assign that variable. Some other matchers available:

assign_contract('entries#new', :entry, entry)
flash_contract('entries#create', :notice, 'Great Success!') if flash_enabled

In these two cases, the method both does the side effect (assigning a variable for a view spec or setting a flash message), and also creates a matching contract. There isn't a corresponding fulfillment matcher for anything else yet, so you have to fulfill them manually. I do this like so, in my controller spec:

describe 'get :new' do
  it { fulfills 'entries#new assign @entry'   }
  it do 
  	# actual test which reflects this fulfillment

Free-form contracts

Sometimes I think of things that need a contract that I have no matchers for, and all I really want is a smart comment. I'm using this for a routing contract relationship now. In that case, you can do this:

 # this is a contract that might be created
 # by a link in a view spec for example
 contract("get / routes")

And fulfill it with

it { fulfills('get / routes') }

All that matters for the contract to be fulfilled is that the string matches, so in this case contractinator is almost acting as merely a smart comment.


After checking out the repo, run bin/setup to install dependencies. Then, run rake spec to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to


Bug reports and pull requests are welcome on GitHub at[USERNAME]/contractinator. This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the Contributor Covenant code of conduct.


The gem is available as open source under the terms of the MIT License.


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