examples/mongodb/update_array_element.txt
> db.users.update({ name: "Zorg", "kids.name" : 'Marry' },
         { $set : { "kids.$.name" : "Mary" } })

> db.users.find()
{ "_id" : ObjectId("526b8fdba444e81f1ca06ba2"), "name" : "Foo" }
{ "_id" : ObjectId("526b9048a444e81f1ca06ba3"), "name" : "Zorg",
     "kids" : [ { "name" : "Joe" }, { "name" : "Mary" } ] }
{ "_id" : ObjectId("526ba048a444e81f1ca06ba4"), "name" : "Moo" }