db={"account":[{_id:23131231,name:"johnDoen"}],"project":[{projectCreatorId:23131231,projectName:"Try"}]}

db.account.aggregate([{"$lookup":{"from":"project","localField":"_id","foreignField":"projectCreatorId","as":"result"}},{"$set":{"result":{"$arrayElemAt":["$result",0]}}},{"$project":{"_id":0,"name":1,"projectName":"$result.projectName","projectCreatorId":"$result.projectCreatorId"}}])