In MongoDB how do you index an embedded object fields in an array?

Question

The mongodb documentation for multikeys gives an example of querying embedded object fields in an array:

http://www.mongodb.org/display/DOCS/Multikeys

But there\

Answer 1

you create the index as if you would with a "normal" field;

db.[collection].ensureIndex( { [yourArrayField] : 1 } )

Answer 2

You can create the following index :

db.posts.ensureIndex({"comments.author" : 1})

This will index only the author field of the embedded documents. Note that the index will be used for

db.posts.find( { "comments.author" : "julie" } )

As well as

db.posts.find( { comments: {$elemMatch: {author : "julie" }}} )