mongo_doc.json

{
	name : "Something",
	things : [
		{foo : "148091a0719c", bar: 1},
		{foo : "0026080741c6", bar: 0}
	]
}

shell output

> db.test.find()                                                                  
{ "_id" : ObjectId("4c3e933ccb3fdf086d596bd0"), "name" : "the_name", "things" : [
	{
		"foo" : "foo1",
		"bar" : 4
	},
	{
		"foo" : "foo2",
		"bar" : 3
	},
	{
		"foo" : "foo3",
		"bar" : 3
	},
	{
		"foo" : "foo4",
		"bar" : 3
	}
] }
> db.test.update({"name": "the_name", "things.foo": "foo2"},{"$set": {"things.$.bar": 12}})
can't append to array using string field name
> 

things.rb

require 'rubygems'
require 'mongo'

def add_name coll, nam
  coll.update(
    {"name" => nam, "things" => {"$size" =>0}}, 
    {"name" => nam, "things"=> []}, 
    :upsert => true)
end

def add_foo coll, nam, foo
  add_name coll, nam
  coll.update(
    {"name" => nam, "things" => {"$not" => {"$elemMatch" => {"foo" => foo}}}}, 
    {"$addToSet" => {"things" => {"foo" => foo, "bar" => 0}}})  
end

def is_foo? coll, nam, foo
  !!coll.find_one({"name" => nam, "things" => {"$elemMatch" => {"foo" => foo}}})
end

def increment_bar coll, nam, foo, inc=1
  return false unless is_foo?(coll, nam, foo)
  coll.update(
    {"name" => nam, "things" => {"$elemMatch" => {"foo" => foo}}}, 
    {"$inc" => {"things.$.bar" => inc}})  
end  


def set_level coll, name, lvl
  s = coll.find_one({"name" => name})
  count = s["things"].count
  size, mod = lvl.divmod(count)
  coll.find().each { |doc| puts doc.inspect }
     
  s["things"].slice(0, mod).each do |thing|
    coll.update(
      {"name" => name, "things" => {"$elemMatch" => {"foo" => thing["foo"]}}}, 
      {"$set" => {"things.$.bar" => size + 1}}
    )  
  end
  s["things"].slice(mod, count).each do |thing|
    coll.update(
      {"name" => name, "things" => {"$elemMatch" => {"foo" => thing["foo"]}}}, 
      {"$set" => {"things.$.bar" => size}}
    )  
  end  
end


connect    = Mongo::Connection.new()
db         = connect.db("test")
collection = db.collection("test")

collection.remove()
collection.create_index([["name", Mongo::ASCENDING]], :unique=> true)
collection.create_index([["things.foo", Mongo::ASCENDING]], :unique=> true)

add_foo collection, "the_name", "foo1"
add_foo collection, "the_name", "foo2"
add_foo collection, "the_name", "foo3"
add_foo collection, "the_name", "foo1"
add_foo collection, "the_name", "foo4"

collection.find().each { |doc| puts doc.inspect }

set_level(collection, "the_name", 13)

collection.find().each { |doc| puts doc.inspect }