##book.rb
has_many :reviews, :dependent => :delete_all do 
    def overall_rating(record)
      to_a.sum(&record)
    end
  end

##irb

>> b = Book.find(2)
=> #<Book id: 2, firstname: "Zach", lastname: "Wentz", approved: true, overall: nil, rating1: nil, rating2: nil, rating3: nil, rating4: nil, created_at: "2008-05-23 17:18:18", updated_at: "2008-05-24 14:16:30">
>> b.reviews.overall_rating(:overall)
=> 5