Mocha: Why can't I stub #caller method?

Question

using Mocha in simple code turned in unexpected way. Could you explain what is going wrong?

require 'test-unit'
require 'mocha'

class A
  def m
    caller.first
  end
end

So using this simple class, we can get the latest caller:

A.new.m #=> "(irb):32:in `irb_binding'" (for example)

But if I want to stub caller call, things going wrong.

a = A.new
a.stubs(:caller)

Mocha::ExpectationError: unexpected invocation: #<A:0x6aac20>.caller()

My guess is to check out Mocha sources, but I will do it later ;)

---

• ruby 1.9.3p194
• test-unit 2.5.0, 2.4.8
• mocha 0.12.6

Answer 1

This is a partial explanation, but I hope it is still useful.

As you've suggested, a way to understand what's going on here is to check the Mocha sources. I think the key to the issue is that the Expectation class, which is used when creating the stub, makes use of the caller method itself.

A workaround would be to use alias_method e.g.

class A
  alias_method :my_caller, :caller # allow caller to be stubbed

  def m
    my_caller.first
  end
end

a = A.new
a.stubs(:my_caller)

Answer 2

You are calling caller from m. So caller.first will always be the line that calls m, which is probably useless. Probably what you wanted is caller[1], not caller.first (or caller[0]).

Answer 3

How about this?

allow_any_instance_of(Kernel)
  .to receive(:caller)
  .and_return(["anything_you_want.rb"])