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Project: RSpec Mocks 3.12

Publisher: RSpec

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.

Scenarios
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

Last published 3 months ago by Jon Rowe.