class A
  def a(arg)
    b(c(arg))
  end

  def b(arg)
    do_expensive_thing(arg)
  end

  def c(arg)
    do_other_expensive_thing(arg)
  end
end

# OPTION A - stubbing methods of class being tested
#   benefit: don't have know exactly what `do_expensive_thing` does,
#            can just stub it entirely
#   cost: hard to refactor method being tested

test 'A#a' do
  inst = A.new
  arg = 1
  arg2 = 2
  arg3 = 3
  inst.expects(:c).with(arg).returns arg2
  inst.expects(:b).with(arg2).returns arg3
  assert_equal arg3, inst.a(arg)
end

# OPTION 2 - Stub the methods being called lower in the stack
#  benefit: more flexibility with refactoring
#  cost: have to dig around for the exact methods that need to get stubbed

test 'A#a' do
  inst = A.new
  arg = 1
  arg2 = 2
  arg3 = 3
  ObscureClass.allows(:obscure_method).with(arg).returns arg2
  ObscureClass.allows(:other_obscure_method).returns arg3
  assert_equal arg3, inst.a(arg)
end