Mongo sum_by.rb

class Mongo::Collection
  def sum_by_raw(ops)
    reduce_function = "function (obj, prev) { prev.count += (obj.#{ops[:sum_field]} ? obj.#{ops[:sum_field]} : 0); }"
    code = Mongo::Code.new(reduce_function)
    group([ops[:key]].flatten, ops[:filter]||{}, {"count" => 0},code)
  end
  def sum_by(ops)
    sum_by_raw(ops).inject({}) { |h,a| k = ops[:key]; h.merge(a[k] => a['count'])}
  end
end

# @coll.save('a' => 'a', 'b' => 1)
# @coll.save("a" => 'a', 'b' => 3)
# @coll.save('a' => 'b', 'b' => 2)
# 
# @coll.sum_by(:key => 'a', :sum_field => 'b').should == {'a' => 4, 'b' => 2}