CodeSchool Rspec Example gives me NoMethodError: 'undefined method '+' for nil:NilClass'

Question

So I'm going through the CodeSchool Rspec track (I'm on level 4) and I like to rewrite the examples to reinforce what I'm learning. I set up a Dog class that mimics the Zombie class, and ran tests that are identical, but for some reason I'm getting the error:

1) Dog is a genius dog
   Failure/Error: before { dog.learn_trick }
   NoMethodError:
     undefined method `+' for nil:NilClass
   # ./app/models/dog.rb:19:in `learn_trick'
   # ./spec/models/dog_spec.rb:23:in `block (2 levels) in <top (required)>'

2) Dog is not a dummy dog
   Failure/Error: before { dog.learn_trick }
   NoMethodError:
     undefined method `+' for nil:NilClass
   # ./app/models/dog.rb:19:in `learn_trick'
   # ./spec/models/dog_spec.rb:23:in `block (2 levels) in <top (required)>'

I don't understand why, here is the code:

CodeSchool Model:

class Zombie < ActiveRecord::Base
  attr_accessible :name
  validates :name, presence: true

  def eat_brains
    self.iq += 3
  end

  def dummy?
    iq < 3
  end

  def genius?
    iq >= 3
  end
end

My Model

class Dog < ActiveRecord::Base
  validates :name, presence: true

  def learn_trick
    self.iq += 3
  end

  def genius?
    iq >= 3
  end

  def dummy? 
    iq < 3
  end
end

Note: I don't use attr_accessible because I'm using Rails4.

The CodeSchool Specs

describe Zombie do
  let(:zombie) { Zombie.new }
  subject { zombie }

  before { zombie.eat_brains }

  it 'is not a dummy zombie' do
    zombie.should_not be_dummy
  end

  it 'is a genius zombie' do
    zombie.should be_genius
  end
end

My Specs

describe Dog do
  let(:dog) { Dog.new }
  subject { dog }

  before { dog.learn_trick }

  it "is not a dummy dog" do
    dog.should_not be_dummy
  end 

  it "is a genius dog" do
    dog.should be_genius
  end 
end

Can anyone explain why I'm getting the NoMethodError? Also, I know the questions on this site usually learn towards the practical but hopefully understanding why I'm getting this error will help me write more practical tests later. Thanks.

Answer 1

iq must have a value before you can increment that. So you need to do is <% iq = 0 %> before self.iq += 3