db={Attendance:[{title:"Class Name",description:"Description",attendances:[{_id:"60ed395fb06f1254b0771ce1",date:"2021-07-13T06:57:00.000+00:00",records:[{attendance:true,_id:"60ed395fb06f1254b0771ce2",studentId:"60e6c440c133a9b590f510d5"},{attendance:false,_id:"60ed395fb06f1254b0771ce3",studentId:"60e6c4cb7be1ce4b647c546f"}]},{_id:"60ed395fb06f1254b0771ce4",date:"2021-07-14T06:57:00.000+00:00",records:[{attendance:true,_id:"60ed395fb06f1254b0771ce5",studentId:"60e6c440c133a9b590f510d5"},{attendance:true,_id:"60ed395fb06f1254b0771ce6",studentId:"60e6c4cb7be1ce4b647c546f"}]}]}],Students:[{_id:"60e6c440c133a9b590f510d5",name:"name1"},{_id:"60e6c4cb7be1ce4b647c546f",name:"name2"}]}

db.Attendance.aggregate([{$addFields:{students:{$reduce:{input:"$attendances",initialValue:[],in:{$setUnion:["$$value","$$this.records.studentId"]}}}}},{$lookup:{from:"Students",localField:"students",foreignField:"_id",as:"students"}},{$project:{title:1,description:1,attendances:{$map:{input:"$attendances",as:"a",in:{$mergeObjects:["$$a",{records:{$map:{input:"$$a.records",as:"r",in:{attendance:"$$r.attendance",_id:"$$r._id",student:{$arrayElemAt:[{$filter:{input:"$students",cond:{$eq:["$$this._id","$$r.studentId"]}}},0]}}}}}]}}}}}])