db={"Register":[{"_id":"Reg_1","userId":"User_1","accessToken":"a_1"},{"_id":"Reg_2","userId":"User_2","accessToken":"a_2"}],"my_Notifications":[{"_id":"N1_1","accessToken":"a_1","summaryId":1111,"dontWantThis1":61,"dontWantThis2":62,"dateCreated":"2020-04-17T00:00:00.000+00:00"},{"_id":"N2_1","accessToken":"a_2","summaryId":2221,"dontWantThis1":71,"dontWantThis2":72,"dateCreated":"2020-04-18T00:00:00.000+00:00"},{"_id":"N1_2","accessToken":"a_1","summaryId":1112,"dontWantThis1":611,"dontWantThis2":622,"dateCreated":"2020-05-25T00:00:00.000+00:00"},{"_id":"N2_2","accessToken":"a_2","summaryId":2222,"dontWantThis1":711,"dontWantThis2":722,"dateCreated":"2020-05-26T00:00:00.000+00:00"},]}

db.Register.aggregate([{$lookup:{from:"my_Notifications",localField:"accessToken",foreignField:"accessToken",as:"notifications"}},{$project:{"_id":1,"userId":1,"accessToken":1,notifications:{$map:{input:"$notifications",as:"n",in:{"_id":"$$n._id","summaryId":"$$n.summaryId","dateCreated":"$$n.dateCreated"}}}}},{$project:{"userId":1,"accessToken":1,"notifications":{$filter:{input:"$notifications",as:"item",cond:{$and:[{$gte:["$$item.dateCreated","2020-04-01T00:00:00.000+00:00"]},{$lte:["$$item.dateCreated","2020-05-31T00:00:00.000+00:00"]}]}}}}},{$unwind:"$notifications"},{$sort:{"notifications.dateCreated":-1}},{$group:{"_id":{"accessToken":"$accessToken","userId":"$userId"},"notifications":{$push:"$notifications"},"notifications_count":{$sum:1}}}])