> db.users.find({created_at: null})
{ "_id" : ObjectId("4e8d0d420ec65305be4f6567"), "username" : "Otgoo", "password" : "foobar" }
{ "_id" : ObjectId("4e8d0d9f0ec65305be4f6569"), "username" : "Tuvhsuu", "created_at" : null }
> db.users.find({created_at: {$type: 10}})
{ "_id" : ObjectId("4e8d0d9f0ec65305be4f6569"), "username" : "Tuvhsuu", "created_at" : null }
> db.users.find({created_at: {$exists: false}})
{ "_id" : ObjectId("4e8d0d420ec65305be4f6567"), "username" : "Otgoo", "password" : "foobar" }