In my adventures through RubyLand, I’ve been using the excellent RR test double framework together with RSpec, for some really expressive tests. The other day I was writing a test to prove that a class had invoked a shell command with the correct command-line. I knew there would be some way of mocking the backtick operator in Ruby, but for once RR was working against me. I found this post on the astrails blog to be the key.

It wasn’t quite enough for me though. I like to test in an AAA style, and what I wanted was the following:

describe "My class" do
  before do
    # Set up stubs
    # Invoke the action
  end

  it "should call some web service" do
    # Simple assertion
  end

  it "should invoke the shell command" do
    # How do I make this assertion?
  end
end

Using the information I already knew, I could set up a mock which would automatically fail if the command wasn’t invoked, but I didn’t want that embedded in the before block. If I changed it to a stub, which would accept any command, how could I verify it later on?

Digging into the code of RR revealed that calling my_object.should have_received.my_method() simply uses method_missing on the InvocationMatcher class. I could invoke that directly and make an assertion on the command that should have been run:

before do
  stub(@target).__double_definition_create__.call(:`) { "some fake output" }
  @target.do_action
end

it "should invoke the shell command" do
  expected_command = "echo 'this is awesome'"
  @target.should have_received.method_missing(:`, expected_command)
end