test_spec.rb

class MyArray < Array
  def my_check
    if size > 4
      :ok
    else
      :too_small
    end
  end

  def my_compact(use_check)
    if use_check && my_check != :ok
      self
    else
      compact
    end
  end
end

describe Array do
  subject { MyArray.new([1, nil, nil]) }

  before :each do
    subject.should_receive(:my_check).and_return(:ok)
  end

  it "doesn't remove nils for small arrays" do
    subject.my_compact(true)
    subject.should == [1, nil, nil]
  end

  context "but without use_check" do
    it "removes nils even for small arrays" do
      subject.should_not_receive(:my_check)
      subject.my_compact(false)
    end
  end
end

# $ rspec test_spec.rb
# .F
#
# Failures:
#
#   1) Array but without use_check removes nils even for small arrays
#      Failure/Error: subject.should_receive(:my_check).and_return(:ok)
#        ([1, nil, nil]).my_check(any args)
#            expected: 1 time with any arguments
#            received: 0 times with any arguments
#      # ./test_spec.rb:23:in `block (2 levels) in <top (required)>'
#
# Finished in 0.00064 seconds
# 2 examples, 1 failure
#
# Failed examples:
#
# rspec ./test_spec.rb:32 # Array but without use_check removes nils even for small arrays