Indexing of Object keys in MongoDB

I have this structure

{
    "_id": "willwill",
    "rental": {
        "hitchhikergalaxy": {[...]},
        "animalfarm": {[..]}
    }
}

Which [..] is a embedded document storing the details of the rental (and the exactly same data also exists on another collection) and the rental Object's keys are _id of the book collection.

How do I ensureIndex() this query?

db.users.find({"rental.animalfarm": {'$exists': true}})

(I use MongoDB on PHP, but the example above is in JavaScript)

You might want to think about refactoring the document such that:

"rental": 
  [
     { "name":"hitchikergalaxy", "attributes": { your stuff } },
     { "name":"animalfarm", "attributes": { your stuff } }
  ]

Now you have a path to ensureIndex "rental.name"

You can further find all documents where any user rented animalfarm using:

db.users.find( { "rental.name": "animalfarm" } )

I've been down the path of using data as keys/names -- and it always seems to make things more complicated than they need to be.