db={collection:[{"_id":"6808990bb8324c2b80635375","notificationId":null,"schedulerId":"68075d9ab426014f20df5f56"},{"_id":"6808a1c8fb203039b0f293b0","notificationId":"68075d9ab426014f20df5f56","schedulerId":null},{_id:"WARNING: has both",notificationId:"68075d9ab426014f20df5f56",schedulerId:"68075d9ab426014f20df5f56"}],notifications:[{_id:"68075d9ab426014f20df5f56",title:"some notification"}],schedulers:[{_id:"68075d9ab426014f20df5f56",description:"many times"}]}

db.collection.aggregate([{$match:{notificationId:{$type:"string"}}},{$lookup:{from:"notifications",localField:"notificationId",foreignField:"_id",as:"notification"}},{$unionWith:{coll:"collection",pipeline:[{$match:{schedulerId:{$type:"string"}}},{$lookup:{from:"schedulers",localField:"schedulerId",foreignField:"_id",as:"schedule"}}]}},{$sort:{_id:1}}])