db={"carDatas":[{carData:[{vin:123456789},{vin:987654321}]}],"vins":[{"_id":123456789,"sku":"almonds"},{"_id":987654321,"sku":"bread"}]}

db.carDatas.aggregate([{$unwind:"$carData"},{$lookup:{from:"vins",localField:"carData.vin",foreignField:"_id",as:"inventory_docs"}}])