How do I test the rescue block of a method with rspec mocks 3.3

Question

Help me make this test pass:

Here is an example of some rspec code,

class User
  attr_accessor :count
  
  def initialize
    @count = 0
  end

  # sometimes raises
  def danger
    puts "IO can be dangerous..."
  rescue IOError => e
    @count += 1
  end
  
  #always raises
  def danger!
    raise IOError.new    
  rescue IOError => e
    @count += 1
  end
end

describe User do  
  describe "#danger!" do
    it "its rescue block always increases the counter by one" do
      allow(subject).to receive(:'danger!')
      
      expect {
        subject.danger!
      }.to change(subject, :count).by(1)
    end
  end

  describe "#danger" do
    context "when it rescues an exception" do
      it "should increase the counter" do
        allow(subject).to receive(:danger).and_raise(IOError)
        
        expect {
          subject.danger
        }.to change(subject, :count).by(1)
      end      
    end
  end
end

I've also created a fiddle with these tests in it, so you can just make them pass. Please help me test the rescue block of a method!

---

Background:

My original question went something like this:

I have a method, like the following:

def publish!(resource)
  published_resource = resource.publish!(current_project)

  resource.update(published: true)

  if resource.has_comments?
    content = render_to_string partial: "#{ resource.class.name.tableize }/comment", locals: { comment: resource.comment_content_attributes }

    resource.publish_comments!(current_project, published_resource.id, content)
  end

  true

  rescue Bcx::ResponseError => e
    resource.errors.add(:base, e.errors)

    raise e
  end

And I want to test that resource.errors.add(:base, e.errors) is, in fact, adding an error to the resource. More generally, I want to test the rescue block in a method.

So I'd like to write code like,

it "collects errors" do 
  expect{ 
    subject.publish!(training_event.basecamp_calendar_event)
  }.to change(training_event.errors.messages, :count).by(1)
end

Of course, this raises an error because I am re-raising in the rescue block.

I've seen a few answers that use the old something.stub(:method_name).and_raise(SomeException), but rspec complains that this syntax is deprecated. I would like to use Rspec Mocks 3.3 and the allow syntax, but I'm having a hard time.

Answer 1

allow(something).to receive(:method_name).and_raise(SomeException)

would be the new allow syntax. Check out the docs for reference.

Answer 2

I was misunderstanding what the allow syntax is actually for. So to make my example specs pass, I needed to do this:

describe "#danger" do
  context "when it rescues an exception" do
    it "should increase the counter" do
      allow($stdout).to receive(:puts).and_raise(IOError) # <----- here

      expect {
        subject.danger
      }.to change(subject, :count).by(1)
    end      
  end
end

This thing that I'm stubing is not the method, or the subject, but the object that might raise. In this case I stub $stdout so that puts will raise.

Here is another fiddle in which the specs are passing.