db={"spotmodel":[{"code":"AD01","streetId":ObjectId("5c16d3e6e6cbea23a0393b4b")},{"code":"AD02","streetId":ObjectId("5c16d3e6e6cbea23a0393b4b")}],"areamodel":[{"description":"Area A","streets":[{"_id":ObjectId("5c16d3e6e6cbea23a0393b4b"),"name":"Waterloo"},{"_id":ObjectId("5c16d536e6cbea23a0393b51"),"name":"Bristol"}]},{"description":"Area B","streets":[{"_id":ObjectId("5c16d3e6e6cbea23a0393b2a"),"name":"Park Av."},{"_id":ObjectId("5c16d536e6cbea23a0393b2b"),"name":"Nelson St."}]}]}

db.areamodel.aggregate([{$unwind:"$streets"},{$lookup:{from:"spotmodel",localField:"streets._id",foreignField:"streetId",as:"codes"}},{$addFields:{"streets.codes":"$codes.code"}},{$group:{_id:"$_id",streets:{$push:"$streets"},description:{$first:"$description"}}}])