db={"Register":[{"_id":"5eac9e815fc57b07f5d0d29f","userId":"5ea108babb65b800172b11be","accessToken":"111"},{"_id":"5ecaeba3c7b910d3276df839","userId":"5e6c2dddad72870c84f8476b","accessToken":"222"}],"my_Notifications":[{"_id":"5ebf0390c719e60004f42e74","accessToken":"111","summaryId":1111,"dontWantThis1":61,"dontWantThis2":62,"dateCreated":"2020-04-17T00:00:00.000+00:00"},{"_id":"6ebf0390c719e60004f42e76","accessToken":"222","summaryId":2221,"dontWantThis1":71,"dontWantThis2":72,"dateCreated":"2020-04-18T00:00:00.000+00:00"},{"_id":"6ebf0390c719e60004f42e78","accessToken":"111","summaryId":1112,"dontWantThis1":611,"dontWantThis2":622,"dateCreated":"2020-05-25T00:00:00.000+00:00"},{"_id":"6ebf0390c719e60004f42e80","accessToken":"222","summaryId":2222,"dontWantThis1":711,"dontWantThis2":722,"dateCreated":"2020-05-26T00:00:00.000+00:00"},]}

db.Register.aggregate([{$lookup:{from:"my_Notifications",localField:"accessToken",foreignField:"accessToken",as:"notifications"}},{$unwind:"$notifications"},{$sort:{"notifications.dateCreated":-1}},{$group:{"_id":{"accessToken":"$accessToken","userId":"$userId"},"notifications":{$push:"$notifications"},"notifications_count":{$sum:1}}}])