Using an object double

object_double can be used to create a double from an existing "template" object, from which it verifies that any stubbed methods on the double also exist on the template. This is useful for objects that are readily constructable, but may have far-reaching side-effects such as talking to a database or external API. In this case, using a double rather than the real thing allows you to focus on the communication patterns of the object's interface without having to worry about accidentally causing side-effects. Object doubles can also be used to verify methods defined on an object using method_missing, which is not possible with instance_double.

In addition, object_double can be used with specific constant values, as shown below. This is for niche situations, such as when dealing with singleton objects.

Doubling an existing object

Given a file named "spec/user_spec.rb" with:

class User
  # Don't want to accidentally trigger this!
  def save; sleep 100; end
end

def save_user(user)
  "saved!" if user.save
end

RSpec.describe '#save_user' do
  it 'renders message on success' do
    user = object_double(User.new, :save => true)
    expect(save_user(user)).to eq("saved!")
  end
end

When I run rspec spec/user_spec.rb

Then the examples should all pass.

Doubling a constant object

Given a file named "spec/email_spec.rb" with:

require 'logger'

module MyApp
  LOGGER = Logger.new("myapp")
end

class Email
  def self.send_to(recipient)
    MyApp::LOGGER.info("Sent to #{recipient}")
    # other emailing logic
  end
end

RSpec.describe Email do
  it 'logs a message when sending' do
    logger = object_double("MyApp::LOGGER", :info => nil).as_stubbed_const
    Email.send_to('hello@foo.com')
    expect(logger).to have_received(:info).with("Sent to hello@foo.com")
  end
end

When I run rspec spec/email_spec.rb

Then the examples should all pass.