https://jira.mongodb.org/browse/MONGOID-5314 suggests that Mongoid accepts partial_filter_expression option when declaring indexes, however this option is not documented in https://www.mongodb.com/docs/mongoid/master/reference/indexes/ and the reference documentation does not state that in general there is option style conversion performed by Mongoid (i.e. application can use underscored keys that Mongoid does not know about and Mongoid will convert those to camel case for the server).

The scope of this ticket is to:

Document whether options unknown to Mongoid are converted from underscore to camel case
Document partial_filter_expression option in particular OR decide to not document individual options
If individual options are documented, review driver documentation for what other options exist and add them to this reference page in Mongoid.