[{"_id":"1","pn":"01","title":"MyAssembly","partlist":[{"pn":"1234","desc":"myPart1",},{"pn":"5678","desc":"myPart2",}]}]

db.collection.aggregate([{$addFields:{"partlist":{$map:{input:"$partlist",in:{"$mergeObjects":["$$this",{id:"$$this.pn"}]}}}}}])