[{"array":[{"_id":"a","timestamp":"t1","type":"object"},{"_id":"b","timestamp":"t2","type":"object"},{"_id":"c","timestamp":"t3","type":"string"},{"_id":"d","timestamp":"t4","type":"string"},{"_id":"e","timestamp":"t5","type":"boolean"}]}]

db.collection.aggregate([{"$unwind":{"path":"$array","includeArrayIndex":"index"}},{"$addFields":{"array.index":{"$toInt":"$index"}}},{"$group":{"_id":"$_id","array":{"$push":"$$ROOT.array"}}}])