Create a database where there are multiple fields: > db.test.find() { "_id" : ObjectId( "5d65896a4aea8c58f56007c2" ), "a" : 1 } { "_id" : ObjectId( "5d6589774aea8c58f56007c3" ), "a" : 2 } { "_id" : ObjectId( "5d65897e4aea8c58f56007c4" ), "a" : 3 } { "_id" : ObjectId( "5d6589864aea8c58f56007c5" ), "a" : 4 } { "_id" : ObjectId( "5d6591f747ccc30d70199b18" ), "a" : { "x" : "z" } } { "_id" : ObjectId( "5d6599ac47ccc30d70199b19" ), "a" : 15, "b" : 99 } { "_id" : ObjectId( "5d659a1347ccc30d70199b1a" ), "a" : 15, "b" : 300 } Create a sort command containing the "$meta" : "sortKey" projection: > findCommand { "find" : "test" , "sort" : { "a" : -1, "b" : -1 }, "projection" : { "sortKey" : { "$meta" : "sortKey" } } } Now the fields in the returned documents are sorted as asked in the query, but the 'sortKey' field only has some of the information used to sort: > db.runCommand(findCommand) { "cursor" : { "firstBatch" : [ { "_id" : ObjectId( "5d6591f747ccc30d70199b18" ), "a" : { "x" : "z" }, "sortKey" : { "" : { "x" : "z" } } }, { "_id" : ObjectId( "5d659a1347ccc30d70199b1a" ), "a" : 15, "b" : 300, "sortKey" : { "" : 15 } }, { "_id" : ObjectId( "5d6599ac47ccc30d70199b19" ), "a" : 15, "b" : 99, "sortKey" : { "" : 15 } }, { "_id" : ObjectId( "5d6589864aea8c58f56007c5" ), "a" : 4, "sortKey" : { "" : 4 } }, { "_id" : ObjectId( "5d65897e4aea8c58f56007c4" ), "a" : 3, "sortKey" : { "" : 3 } }, { "_id" : ObjectId( "5d6589774aea8c58f56007c3" ), "a" : 2, "sortKey" : { "" : 2 } }, { "_id" : ObjectId( "5d65896a4aea8c58f56007c2" ), "a" : 1, "sortKey" : { "" : 1 } } ], "id" : NumberLong(0), "ns" : "test.test" }, "ok" : 1 } Now the sort is occurring on both 'a' and 'b' but only the values of 'a' are included in the sortKey field.