This is based on an example I saw while watching the Aaron Patterson Play by Play from Peepcode. The syntax is really nice.

records = [ 
            { :id => 1, :score => 1 }, 
            { :id => 2, :score => 4 }, 
            { :id => 3, :score => 9 } 
          ]
 
records.map { |record| record[:score] }.reduce(:+) # => 14

This will work will an collection such as an Array of ActiveRecord objects.