db={"user":[{"user":{"id":1001,username:"dilag",roster:[{ucid:"r0s122",name:"sam"},{ucid:"r0s123",name:"ram"},{ucid:"r0s124",name:"rat"}]}}],"unread":[{ucid:"r0s122",usernameAndCount:[{username:"dilag",count:100},{username:"ramg",count:20}]},{ucid:"r0s123",usernameAndCount:[{username:"dilag",count:100},{username:"ramg",count:20}]}]}

db.unread.aggregate([{"$lookup":{from:"user",let:{uid:"$ucid"},pipeline:[{"$unwind":"$user.roster"},{$match:{$expr:{$eq:["$user.roster.ucid","$$uid"]}}}],as:"users"}},{"$addFields":{name:{$ifNull:[{"$arrayElemAt":["$users.user.roster.name",0]},""]}}},{$project:{users:0}}])