[{"_id":ObjectId("5a934e000102030405000000"),"description":"description for item 1","item_code":"00001"},{"_id":ObjectId("5a934e000102030405000001"),"description":"description for item 2","item_code":"00002"},{"_id":ObjectId("5a934e000102030405000002"),"description":"description for item 3","item_code":"00003"},{"_id":ObjectId("5a934e000102030405000003"),"extrafield":"extra field for item 2","item_code":"00002"}]

db.collection.aggregate([{$group:{_id:"$item_code",root:{$mergeObjects:"$$ROOT"}}},{$replaceWith:"$root"}])