The difference between RSpec stubs and mocks

Ruby on Rails / RSpec

You may want to check how to use stubs and mocks with RSpec before diving in into this subject.

Syntax differences

The only one syntax difference is the first method. With stubs, you would use allow while with mocks you would use expect

Usage differences

Stubs are used to avoid execution of other methods and make them return desired values. Mocks are used to check if given method was executed with desired arguments and frequency.

Example

It’s always better to learn by the example so let’s use one. We will create class and sample test for it:

class Message
  def initialize(user: user)
    @user = user
  end

  def send
    user.update_message_sent_at
    name = user.name(format: 'message')
    Emailer.send_message(name)
  end

  private
  attr_reader :user
end

We don’t really want to call any of those three methods so we will stub them and then use mocks to check if they were executed properly.

require 'spec_helper'

describe Message do
  describe '#send' do
    it 'sends message' do
      user = instance_double(User)
      name = 'Tim'
      allow(user).to receive(:name).with(format: 'message').and_return(name)
      allow(user).to receive(:update_message_sent_at)
      alloW(Emailer).to receive(:send_message).with(name)

      message = Message.new(user: user)
      message.send

      expect(user).to have_received(:update_message_sent_at).once
      expect(user).to have_received(:name).with(format: 'message').once
      expect(Emailer).to have_received(:send_message).with(name).once
    end
  end
end

And that’s it! As you can see we used stubs to not execute original code and used mocks to check if given method was really invoked. Such test structure is named as spies.