In https://jira.mongodb.org/browse/MONGOID-5314 it was suggested that the index declaration takes into account storage field names (https://www.mongodb.com/docs/mongoid/master/reference/fields/#specifying-storage-field-names). This is not documented one way or another in https://www.mongodb.com/docs/mongoid/master/reference/indexes/.

The scope of this ticket is to:

Confirm whether storage field names are taken into account.
Verify there is test coverage for this (in either case of behavior).
Add documentation stating the behavior to https://www.mongodb.com/docs/mongoid/master/reference/indexes/.