In the sparse index documentation I found note about mongodb 3.2 partial indexes
Changed in version 3.2: Starting in MongoDB 3.2, MongoDB provides the option to create partial indexes. Partial indexes offer a superset of the functionality of sparse indexes. If you are using MongoDB 3.2 or later, partial indexes should be preferred over sparse indexes.
Partial indexes are very helpfull and I want to use them in my project. Is it possible use them with mongoose?