Interesting little rSpec gotcha

This one had Adam and me stumped for a while. Trying to check that a method is only called on objects that respond to it:

describe "Foo#call_all_the_things" do
  let(:foo_1) { stub :foo_1, bar: "hello" }
  let(:foo_2) { stub :foo_2 }
  subject { Foo.new foo_1, foo_2 }

  it "only calls bar on objects that respond to it" do
    foo_1.should_receive :bar
    foo_2.should_not_receive :bar
    subject.call_all_the_things(:bar)
  end
end

class Foo
  def initialize *things
    @things = things
  end

  def call_all_the_things method
    @things.each do |thing|
      thing.send method if thing.respond_to? method
    end
  end
end


  1) Foo#call_all_the_things only calls bar on objects that respond to it
     Failure/Error: thing.send method if thing.respond_to? method
       (Stub :foo_2).bar(no args)
           expected: 0 times
           received: 1 time

Hmm. Why is it calling bar on the thing that doesn’t respond to it? Perhaps rSpec doubles don’t handle respond_to? properly?

[1] pry(main)> require "rspec/mocks/standalone"
=> true
[2] pry(main)> foo = stub foo: 123
=> #
[3] pry(main)> foo.respond_to? :foo
=> true
[4] pry(main)> foo.respond_to? :bar
=> false

Nope.

FX: lightbulb above head

Of course! To do the should_not_receive check, it needs to stub the method, which means it responds to it!

Two possible solutions: either let the fact that the missing method isn’t called be tested implicitly, or specify that when objects that don’t respond to the method exist, no NoMethodError is raised.