[{"collection":"collection","count":50,"content":{"_id":{"type":"autoincrement","autoType":"int","startInt":1},"id":{"type":"int","minInt":1,"maxInt":50}}}]

db.collection.aggregate([{"$group":{"_id":null,"maxId":{"$max":"$id"},"idSet":{"$addToSet":"$id"}}},{"$set":{"missingIds":{"$setDifference":[{"$range":[1,"$maxId"]},"$idSet"]}}},{"$project":{"_id":0,"maxId":1,"missingIds":1}}])