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book.rb 


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

				
		

		

	
	





irb 





	
	

		

		

			
				
				

					
		
		
>> b = Book.find(2)
=> #
>> b.reviews.overall_rating(:overall)
=> 5