Database
db={"a":[{"UniqeId":1,"Hobbies":"Eating"},{"UniqeId":2,"Hobbies":"Sleeping"},{"UniqeId":3,"Hobbies":"Walking"}],"b":[{"UserName":"Sukuna","UserType":"Villan"}]}
Query
db.a.aggregate([{$lookup:{from:"b",localField:"1",foreignField:"1",as:"docs"}},{$replaceRoot:{newRoot:{$mergeObjects:["$$ROOT",{$first:"$docs"}]}}},{$unset:["docs","_id"]},{$group:{_id:"$UserName",doc:{$first:"$$ROOT"}}},{$replaceWith:"$doc"}])