failing_test.rb

class Wheel
    attr_reader :rim, :tire

    def initialize(rim, tire)
        @rim  = rim
        @tire = tire
    end

    def diameter
        rim + (tire * 2)
    end
end

class Gear
    attr_reader :chainring, :cog, :wheel, :observer

    def initialize(args)
        @chainring = args[:chainring]
        @cog       = args[:cog]
        @wheel     = args[:wheel]
        @observer  = args[:observer]
    end

    def gear_inches
        ratio * wheel.diameter
    end

    def ratio
        chainring / cog.to_f
    end

    def set_chainring(new_chainring)
        @chainring = new_chainring
        changed
    end

    def set_cog(new_cog)
        @cog = new_cog
        changed
    end

    def changed
        observer.changed(chainring, cog)
    end
end

require "minitest/autorun"
class WheelTest < MiniTest::Test

    def setup
        @wheel = Wheel.new(26, 1.5)
    end

    def test_implements_the_diameterizable_interface
        assert_respond_to(@wheel, :diameter)
    end

    def test_calculates_diameter
        wheel = Wheel.new(26, 1.5)

        assert_in_delta(29,
                        wheel.diameter,
                        0.01)
    end
end

# Create a player of the 'Diameterizable' role.
class DiameterDouble

    def diameter
        10
    end
end

class GearTest < MiniTest::Test

    def setup
        @wheel = @diameterizable = DiameterDouble.new
    end

    def test_calculates_gear_inches
        gear = Gear.new(
            chainring: 52,
            cog:       11,
            wheel:     @wheel)

        assert_in_delta(47.27,
                        gear.gear_inches,
                        0.01)
    end
end

class GearTest < MiniTest::Test

    def setup
        @observer = MiniTest::Mock.new
        @gear     = Gear.new(
            chainring: 52,
            cog:       11,
            observer:  @observer)
    end

    def test_notifies_observers_when_cogs_change
        @observer.expect(:changed, true, [52, 27])
        @gear.set_cog(27)
        @observer.verify
    end

    def test_notifies_observers_when_chaings_change
        @observer.expect(:changed, true, [42, 11])
        @gear.set_chainring(42)
        @observer.verify
    end
end

output.sh

Run options: --seed 38813

# Running:

..E..

Finished in 0.001324s, 3776.4350 runs/s, 1510.5740 assertions/s.

  1) Error:
GearTest#test_calculates_gear_inches:
NoMethodError: undefined method `diameter' for nil:NilClass
    failing_test.rb:25:in `gear_inches'
    failing_test.rb:88:in `test_calculates_gear_inches'

5 runs, 2 assertions, 0 failures, 1 errors, 0 skips