db={"bookSchema":[{_id:ObjectId("637d05dc32428ed75ea08d09"),book_details:{book_name:"random123",book_auth:"Amber"},},{_id:ObjectId("637d0673ce0f17f6c473dee2"),book_details:{book_name:"random321",book_auth:"Amber"},},{_id:ObjectId("637d069a3d597c8458ebe4ec"),book_details:{book_name:"random676",book_auth:"Amber"},},{_id:ObjectId("637d06c05b32d503007bcb54"),book_details:{book_name:"random999",book_auth:"Saurav"},},]}

db.bookSchema.aggregate([{$group:{_id:"$book_details.book_auth",count:{$sum:1}}},{"$group":{"_id":0,data:{$push:{k:{"$concat":["score_",{"$toLower":"$_id"}]},v:{"$sum":"$count"}}}}},{"$replaceRoot":{"newRoot":{"$arrayToObject":"$data"}}}])