db={"users":[{"_id":ObjectId("5e9e4af8d042ba24c83763be"),"blacklistGroup":["5e99fd3aa506cf570056898d","5e99fde5a506cf5700568991","5e99fd64a506cf570056898e","5e98c950f4fb3f63b4634e30","5e99fd15a506cf570056898c","5e99fda5a506cf570056898f","5e99fdc7a506cf5700568990","5e99fcf3a506cf570056898b","5e98e90d85e69146f841d23a","5e9867ff5e72550988820dd3","5e98e8e785e69146f841d239"]}],"groups":[{"_id":"5e99fd3aa506cf570056898d","username":"sagar"},{"_id":"5e99fde5a506cf5700568991","username":"shiva"}]}

db.users.aggregate([{"$match":{"_id":ObjectId("5e9e4af8d042ba24c83763be")}},{"$project":{"blacklistGroup":{"$slice":["$blacklistGroup",1]}}},{"$lookup":{"from":"groups","localField":"blacklistGroup","foreignField":"_id","as":"blacklistGroup"}},{"$project":{"usernames":"$blacklistGroup.username"}}])