Selective stubbing of method calls in RSpec

While building a simple API in Rails, I had the following method in my User model that needed to be tested:

def generate_auth_token!
  loop do
    self.auth_token = Devise.friendly_token
    break unless self.class.exists?(auth_token: auth_token)
  end
end

The method uses Devise’s friendly_token method to generate an authentication token for a new user record. After calling the method once, the loop checks whether a user record with the same auth token already exists. If the answer is positive, the method goes for a second try; otherwise it breaks out of the loop, mission accomplished.

To test this, then, it is necessary to simulate the situation where the first call to friendly_token returns an auth token already taken by another record, while a subsequent call does not. Enter the block implementation of RSpec Mocks.

  describe '#generate_auth_token!' do
    it 'generates a new token when one is taken' do
      original_friendly_token = Devise.method(:friendly_token)
      call_count = 0
      allow(Devise).to receive(:friendly_token) do
        if (call_count += 1) == 1
          'veryunique123'
        else
          original_friendly_token.call
        end
      end

      existing_user = FactoryGirl.create :user, auth_token: 'veryunique123'
      user.generate_auth_token!
      expect(user.auth_token).not_to eq 'veryunique123'
    end
  end

On L3, the original implementation of the method being stubbed is assigned to a variable. Within the block, call count (1 or > 1) determines whether the method is stubbed to return a predetermined value or whether the response is delegated to the method’s original implementation. (To avoid even the slightest possibility of a randomly failing test, I could have stubbed the subsequent calls to return a distinct preset value instead of hoping that Devise.friendly_token will never return "veryunique123".)

Obviously, the block’s conditions can be customized to fit the test’s needs, and the entire block implementation can be used for other purposes as well, as you can find out in the RSpec docs.

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