db={"Assets":[{"_id":1,"name":"A","records":[23,24,25,27]}],"Records":[{"_id":23,"Fields":[{"Name":"foo","Value":"bar",}],}]}

db.Assets.aggregate([{$lookup:{from:"Records",localField:"records",foreignField:"_id",as:"match"}},{$unwind:"$match"},{$unwind:"$match.Fields"},{$project:{"Field_name":"$match.Fields.Name",_id:0}}])