rspec: How to stub an instance method called by constructor?

Question

class A
  def initialize
    @x = do_something
  end

  def do_something
    42
  end
end

How can I stub do_something in rspec, before the original implementation is called (thus assigning 42 to @x)? And without changing the implementation, of course.

Answer 1

I don't know how to do that in spec's mock framework, but you can easily swap it out for mocha to do the following:

# should probably be in spec/spec_helper.rb
Spec::Runner.configure do |config|
  config.mock_with :mocha
end

describe A, " when initialized" do
  it "should set x to 42" do
    A.new.x.should == 42
  end
end

describe A, " when do_something is mocked" do
  it "should set x to 23" do
    A.any_instance.expects(:do_something).returns(23)
    A.new.x.should == 23
  end
end

Answer 2

or with RR:

stub.any_instance_of(A).do_something { 23 }

Answer 3

Here's the commit which adds the feature to rspec - This was on May 25 2008

However, the latest gem version of rspec (1.1.11, October 2008) doesn't have this patch in it.

This ticket states that they yanked it out for maintenance reasons, and an alternative solution hasn't yet been provided.

Doesn't look like you can do it at this point. You'll have to hack the class manually using alias_method or somesuch

Answer 4

In my version of rspec (1.2.2) I can do this:

A.should_receive(:new).and_return(42)

I know it is probably to late to answer to the original poster, but I answer it anyway for future reference, as I came here with the same question but figure it out it was working on the latest rspec version.

Answer 5

To stub an instance method, you can do something like this:

before :each do
  @my_stub = stub("A")
  @my_stub.should_receive(:do_something).with(no_args()).and_return(42)
  @my_stub.should_receive(:do_something_else).with(any_args()).and_return(true)
  A.stub(:new).and_return(my_stub)
end

But as pschneider pointed out, just return 42 on new with: A.stub(:new).and_return(42) or something to that effect.

Answer 6

I've found this solution on http://pivots.pivotallabs.com/users/brian/blog/articles/352-introducing-rr

new_method = A.method(:new)

A.stub!(:new).and_return do |*args|
  a = new_method.call(*args)
  a.should_receive(:do_something).and_return(23)
  a
end

Answer 7

I do like Denis Barushev answer. And I'd like to suggest only one cosmetic change which makes new_method variable unnecessary. Rspec does munge on stubbed methods, so that they can be accessed with proxied_by_rspec__ prefix:

A.stub!(:new).and_return do |*args|
  a = A.proxied_by_rspec__new(*args)
  a.should_receive(:do_something).and_return(23)
  a
end