gistfile1.txt

MongoDB shell version: 2.4.8
connecting to: dev
> db.some_collection.ensureIndex({"sub.id":1}, {unique:true});
> db.some_collection.insert({sub:[{id:42}]})
> db.some_collection.find()
{ "_id" : ObjectId("5321a4b172a13ee462bc5c20"), "sub" : [  {  "id" : 42 } ] }
> db.some_collection.update({}, {$push: {sub: {id:42}}})
> db.some_collection.find()
{ "_id" : ObjectId("5321a4b172a13ee462bc5c20"), "sub" : [  {  "id" : 42 },  {  "id" : 42 } ] }
> db.some_collection.insert({sub:[{id:42}]})
E11000 duplicate key error index: dev.some_collection.$sub.id_1  dup key: { : 42.0 }
>