Creating a unique index on a nested array of objects.

index.js

// http://www.mongodb.org/display/DOCS/Multikeys
// Embedded object fields in an array

db.testnest.insert({"title" : "How the west was won", 
 "comments" : [{"text" : "great!" , "author" : "sam"},
               {"text" : "ok" , "author" : "julie"}],
 "_id" : ObjectId("497ce79f1ca9ca6d3efca325")});

// Tried both of these indexes
//db.testnest.ensureIndex({'comments.author': 1}, {unique: true});
db.testnest.ensureIndex({_id: 1, 'comments.author': 1}, {unique: true, dropDupes: true});


// I want this to fail
db.testnest.update({"_id" : ObjectId("497ce79f1ca9ca6d3efca325")}, {$push: {comments: {author: 'sam', text: 'wow'}}});

// this does what I expect, without an index:
db.testnest.update({"_id" : ObjectId("497ce79f1ca9ca6d3efca325"), 'comments.author': {$ne: 'julie'}}, {$push: {comments: {author: 'julie', text: 'I am not posted'}}});


// 'sam' is the author of two comments.
db.testnest.find();
/*
 * Output:

{ "_id" : ObjectId("497ce79f1ca9ca6d3efca325"), "comments" : [
	{
		"text" : "great!",
		"author" : "sam"
	},
	{
		"text" : "ok",
		"author" : "julie"
	},
	{
		"author" : "sam",
		"text" : "wow"
	}
], "title" : "How the west was won" }

*/