gistfile1.rb

map = "function() { emit(this.cargo, {weight: this.weight}); }"
reduce = <<-FUNC
  function(key, values) { 
    var weight = 0; 
    values.forEach(function(doc) { 
      weight += doc.weight 
    }); 
    return({weight: weight}); 
};
FUNC

Shipments.collection.map_reduce(map, reduce, ).find.sort("value.weight").to_a