> db.foo.insert({a: [1, {b: 2}, 3, {}]})  // Note 4 elements in 'a'.
WriteResult({ "nInserted" : 1 })
> db.foo.aggregate([{$project: {"a.b": {$literal: "NEW"}}}]).pretty()
{
	"_id" : ObjectId("57d6d3cd150e60d4d52d9714"),
	"a" : [  // Note there are still 4 elements in 'a'. scalar values have been replaced with documents.
		{
			"b" : "NEW"
		},
		{
			"b" : "NEW"
		},
		{
			"b" : "NEW"
		},
		{
			"b" : "NEW"
		}
	]
}
> db.foo.aggregate([{$project: {"a.b.c": {$literal: "NEW"}}}]).pretty()
{
	"_id" : ObjectId("57d6d3cd150e60d4d52d9714"),
	"a" : [  // Note there are only 2 values in 'a'.
		{
			"b" : {
				"c" : "NEW"
			}
		},
		{
			"b" : {
				"c" : "NEW"
			}
		}
	]
}

> db.foo.explain().aggregate([{$project: {"a.b.c": {$literal: "NEW"}}}])
{
	"stages" : [
		{
			"$cursor" : {
				"query" : {
					
				},
				"fields" : {
					"a.b" : 1,
					"_id" : 1
				},
    ...  // Other explain info.
}
> db.foo.find({}, {_id: 1, "a.b": 1}).pretty()
{
	"_id" : ObjectId("57d6d443f5f2b41fe6bb267c"),
	"a" : [  // Note only 2 of 4 elements result from this projection.
		{
			"b" : 2
		},
		{
			
		}
	]
}