http://blog.davidchelimsky.net/2012/02/12/validations-are-behavior-associations-are-structure/

article.rb

class Article < ActiveRecord::Base
  validates_presence_of :title
  has_many :comments
end

contract.rb

class Contract < ActiveRecord::Base
  validates_presence_of :name
  has_many :monthly_expenses
  calculates_default_monthly_charge
end

rental_contract.rb

class RentalContract < ActiveRecord::Base
  has_many :monthly_charges
  has_one :rentable, :polymorphic => true

  validates_presence_of :rentable

  def default_monthly_charge
    price / months_applied
  end
end

validate_presence_of_title.rb

describe Article do
  it { should validate_presence_of(:title) }
end

with_comments_by.rb

describe Article do
  describe "#with_comments_by" do
    it "finds articles with comments by the submitted comment_author" do
      article = Factory(:article)
      article.comments << Factory.build(:comment, :author => "jdoe")
      Article.with_comments_by("jdoe").should eq([article])
    end
  end
end