db={"coll_1":[{CREATEDBY:"Lorraine Clarkin",CREATIONDATE:"03-MAR-21",BARCODEID:"A1429857",RECORDDESCRIPTION:"Putnam N1A review 10 31 2020 funds1 of 4",CHECKINDATE:"01-JAN-01",RECORDTYPE:"Assurance Workpapers",RECORDSTATE:"Recorded"}],"coll_2":[{"BARCODEID":"A1429857","RECORDCONTENTS":"Signed rep letter"},{"BARCODEID":"A1429857","RECORDCONTENTS":"Reference letter"}]}

db.coll_1.aggregate([{"$lookup":{"from":"coll_2","localField":"BARCODEID","foreignField":"BARCODEID","as":"RECORDCONTENTS"}},{"$set":{"RECORDCONTENTS":{"$map":{"input":"$RECORDCONTENTS","in":"$$this.RECORDCONTENTS"}}}}])