Misha Slyusarev Misha Slyusarev - 5 months ago 32
Ruby Question

RSpec: Allow instance variable to receive method

Say I have this class:

class MyQueue
def initialize
@queue = Queue.new
end
def push(line)
@queue.push(line)
end
end


How should I test that instance variable @queue will receive push method when I call for push on MyQueue instance?

I tried this so far:

describe MyQueue do
let(:my_queue) { instance_double('MyQueue') }
let(:message_line) { double('message_line') }

describe '#push' do
before do
instance_queue = my_queue.instance_variable_set(:@queue, double)
allow(instance_queue).to receive(:push) { message_line }
end

it 'adds line to the queue' do
expect(my_queue.instance_variable_get(:@queue)).to receive(:push) { message_line }
my_queue.push(message_line)
end
end
end


But getting error:

#<InstanceDouble(MyQueue) (anonymous)> received unexpected message :push
with (#<Double "message_line">)


What am I doing wrong?

Answer

You are creating double of MyQueue class but you do not specify that it is allowed to receive #push, that's why it fails on my_queue.push(message_line) line.

In fact, you are going in wrong direction. You want to test a method of the MyQueue class, which uses Queue instance. What you want to do is to stub everything related to the interaction with Queue, but you instead mock MyQueue instance(my_queue) making it dummy(and so, making all calls to it, including #push, which you want to test, dummy).

Here is how I see it should be tested:

describe MyQueue do
  let(:queue) { instance_double('Queue') }
  let(:message_line) { double('message_line') }

  describe '#push' do
    before do
      allow(Queue).to receive(:new).and_return(queue)
      allow(queue).to receive(:push).with(message_line)
    end

    it 'adds line to the queue' do
      expect(queue).to receive(:push).with(message_line)
      MyQueue.new.push(message_line)
    end
  end
end

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