t = Time.now
items = Rating.all(:select => "content_id, viewer_id, viewer_type, rate").inject([]) do |mem, rating|
  item = rating.content_id.to_i
  user = "#{rating.viewer_type}_#{rating.viewer_id}".to_sym
  (mem[item] ||= {})[user] = rating.rate
  mem
end

# iterate over items
items.each_with_index do |pivot_users, pivot|
  next if pivot_users.blank?
  #iterate over the items with higher index
  items.from((pivot+1)..-1).each_with_index do |deviant_users, deviant|
    next if deviant_users.blank?
    common_users = pivot_users.keys + deviant_users.keys
    diff_sum = common_users.inject(0) { |sum, user| sum += pivot_users[user]-deviant_users[user] }
    mean_diff = diff_sum/common_users.size
  end
end
puts (Time.now-t)