gistfile1.txt

Using mongo-0.15.1 with mongo_ext-0.15.1

require 'rubygems' 
require 'mongo' 
 
db = Mongo::Connection.new('localhost', 27017).db('stuff') 
 
coll = db['stuff'] 
 
coll.remove 
coll.save({"_id" => "4ae06a7662125b1b72000001", 'name' => "first doc"}) 
coll.save({'_id' => '4ae06a7662125b1dsf000002', 'name' => "second doc"}) 
coll.save({'name' => "doc without id"})

------
Results:
> db.stuff.find()
{ "_id" : "4ae06a7662125b1b72000001", "name" : "first doc" }
{ "_id" : "4ae06a7662125b1dsf000002", "name" : "second doc" }
{ "_id" : ObjectId("4ae09bae62125b2410000001"), "name" : "doc without id" }

Same behavior without mongo_ext.