Database
[{"_id":ObjectId("5c66a979e109fe0f537c7e37"),"devices":[{"dev_token":"XXXX","_id":ObjectId("5ccc0fa5f7778412173d22bf")}]},{"_id":ObjectId("5c66b6382b18fc4ff0276dcc"),"devices":[{"dev_token":"XXXX","_id":ObjectId("5c93316cc33c622bdcfaa4be")}]}]
Query
db.collection.aggregate([{"$unwind":"$devices"},{$addFields:{"devices.date":{$toDate:"$_id"}}},{"$group":{"_id":"$_id","devices":{"$push":"$devices"}}}])