returns keys (strings) for every path into a schema with a value of type https://www.mongodb.com/docs/manual/tutorial/query-embedded-documents/
returns keys (strings) for every path into a schema with a value of type https://www.mongodb.com/docs/manual/tutorial/query-embedded-documents/