mongo_safe_updates.rb

require 'mongo'

include Mongo

FIRST = 1
SECOND = 2
THIRD = 3
FOURTH = 4

db = Connection.new('localhost').db('test_safe_updates')
coll = db.collection('items')
coll.clear

# create a new object
id = coll.insert({ :state => FIRST })

# update from FIRST to SECOND
from  = { '_id' => id, :state => FIRST }
to    = { '$set' => { :state => SECOND } }
result = coll.update(from, to, :safe => true)      

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

# now try from THIRD (which doesn't exist) to FOURTH
from  = { '_id' => id, :state => THIRD }
to    = { '$set' => { :state => FOURTH } }

# shouldn't this give me an error? or return something other than nil?
result = coll.update(from, to, :safe => true) 

puts db.error
puts coll.find().each { |doc| puts doc.inspect }