db={"users":[{_id:1,userId:123,contributions:"launch 1"},{_id:2,userId:456,contributions:"launch 2"}],"launches":[{_id:"launch 1",name:"Amazing",addedAt:new Date(),description:"The first launch"},{_id:"launch 3"}]}

db.users.aggregate([{$match:{userId:123}},{$lookup:{from:"launches",localField:"contributions",foreignField:"_id",as:"launch"}},{/**Since the $lookup is matching against _id, we know there will only be a maximum of one result*/$unwind:"$launch"},{$project:{user:"$userId",title:"You Added One",launchName:"$launch.name",launchId:"$launch._id",date:"$launch.addedAt",content:"$launch.description",_id:0}}])