Indexes on collections may be created via ensure action:

for example:

creates one index that will be constructed in background : ensure(index("foo" -> Order.Ascending) background true)
   creates one compound index that is sparse :

ensure(index("foo" -> Order.Descending, "doo" -> Order.Ascending) sparse true)

All attributes that can be specified can be found in http://docs.mongodb.org/manual/reference/method/db.collection.ensureIndex/#db.collection.ensureIndex.

Operations: