[fcoury@imac 17:47]:$ mongo
MongoDB shell version: 0.9.6
url: test
connecting to: test
type "help" for help
> db.sample.remove();
> db.sample.save({"_id" : "db1940364a68bbca000000a1" , "test" : [

{"name" : "domain" , "default" : "" , "validation" : "" , "validation_error" : "" , "label" : "Domain" , "required" : false}

]})
> db.sample.update(

{"_id":"db1940364a68bbca000000a1"}

, {$set: {"test.0.new_attribute": "new_value"}})
> db.sample.find();
{"_id" : "db1940364a68bbca000000a1" , "test" : [

{"name" : "domain" , "default" : "" , "validation" : "" , "validation_error" : "" , "label" : "Domain" , "required" : false}

]}