Mongoid is missing the following test coverage for the index macro:

1. Test which cover end-to-end index creation for all options (sparse, unique, partialFilterExpression, etc.) are actually created in the database.
2. tests for index option snake_case aliases, such as partial_filter_expression to partialFilterExpression

These are covered in the MongoDB Ruby Driver, but it would be good add to Mongoid as well. (Fine to close this ticket if team believes MongoDB Ruby Driver tests are sufficient.)