examples/mongodb/update_unset.txt
> db.users.update({ name : 'Zorg' }, { $unset : { kid : '' } } )

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