examples/mongodb/update.txt
> db.users.update({ name : 'Bar' }, { name: 'Zorg'})
> db.users.find()
{ "_id" : ObjectId("526b8fdba444e81f1ca06ba2"), "name" : "Foo" }
{ "_id" : ObjectId("526b9048a444e81f1ca06ba3"), "name" : "Zorg" }
{ "_id" : ObjectId("526ba048a444e81f1ca06ba4"), "name" : "Moo" }

This will replace the entire docuent keeping only the _id.