Partial test doubles

A partial test double is an extension of a real object in a system that is instrumented with test-double like behaviour in the context of a test. This technique is very common in Ruby because we often see class objects acting as global namespaces for methods. For example, in Rails:

person = double("person")
allow(Person).to receive(:find) { person }

In this case we're instrumenting Person to return the person object we've defined whenever it receives the find message. We can also set a message expectation so that the example fails if find is not called:

person = double("person")
expect(Person).to receive(:find) { person }

RSpec replaces the method we're stubbing or mocking with its own test-double like method. At the end of the example, RSpec verifies any message expectations, and then restores the original methods.

Note: we recommend enabling the verify_partial_doubles config option.

Only the specified methods are redefined

Given a file named "partial_double_spec.rb" with:

RSpec.describe "A partial double" do
  # Note: stubbing a string like this is a terrible idea.
  #       This is just for demonstration purposes.
  let(:string) { "a string" }
  before { allow(string).to receive(:length).and_return(500) }

  it "redefines the specified methods" do
    expect(string.length).to eq(500)
  end

  it "does not effect other methods" do
    expect(string.reverse).to eq("gnirts a")
  end
end

When I run rspec partial_double_spec.rb

Then the examples should all pass.

The original method is restored when the example completes

Given a file named "partial_double_spec.rb" with:

class User
  def self.find(id)
    :original_return_value
  end
end

RSpec.describe "A partial double" do
  it "redefines a method" do
    allow(User).to receive(:find).and_return(:redefined)
    expect(User.find(3)).to eq(:redefined)
  end

  it "restores the redefined method after the example completes" do
    expect(User.find(3)).to eq(:original_return_value)
  end
end

When I run rspec partial_double_spec.rb --order defined

Then the examples should all pass.