How can I mock an instance variable in rspec

Question

I have two class OneClass and AnotherClass:

class OneClass
  def initialize(*args)
    @another_member = AnotherClass.new()
  end

  def my_method()
    if @another_member.another_method1() then
      @another_member.another_method2()
    end
    @another_member.another_method3()
  end
end

I am getting to write unit for OneClass. How can I mock @another_member?

Answer 1

With the idea of Anthony, I make it work.

describe OneClass do
  before(:each) { @one_object = OneClass.new }

  describe 'my_method' do
    it 'should work' do
      mock_member = double
      allow(mock_member).to receive(:another_method1).and_return(true)
      @one_object.instance_variable_set(:@another_member, mock_member)

      @one_object.my_method()

      expect(mock_member).to have_received(:another_method1)
    end
  end
end

Answer 2

You can't mock an instance variable. You can only mock methods. One option is to define a method inside OneClass that wraps the another_member, and mock that method.

class OneClass
  def initialize(*args)
  end

  def my_method()
    if another_member.another_method1() then
      another_member.another_method2()
    end
    another_member.another_method3()
  end

  private

  def another_member
    @another_member ||= AnotherClass.new()
  end

end

However, you don't have to, there is a better way to write and test your code. In this case a better approach to mocking is to use a pattern called Dependency Injection.

You pass your dependency to the initializer.

class OneClass
  def initialize(another: AnotherClass, whatever:, somethingelse:)
    @another_member = another.new()
  end

  def my_method()
    if @another_member.another_method1() then
      @another_member.another_method2()
    end
    @another_member.another_method3()
  end
end

(Note I used keyword arguments, but you don't have to. You can also use the standard args approach).

Then, in the test suite you simply provide the test object.

let(:test_another) {
  Class.new do
    def another_method1
      :foo
    end
    def another_method2
      :bar
    end
    def another_method3
      :baz
    end
  end
}

it "does something" do
  subject = OneClass.new(another: test_another)
  # ...
end

There are several advantages of this approach. In particular, you avoid using mock in the tests and you really test the object in isolation.