Database

[{"_id":ObjectId("622136811b68d9136e48ba4e"),"brand_name":"iPhone","brand_rating":[{"cust_name":"Sham K","rating":5},{"cust_name":"Nil","rating":5}],"models":[{"model_name":"iPhone 7Plus","RAM":"4GB","ROM":"64GB","price":98000,"buyer":[{"cust_name":"Anu","rating":3},{"cust_name":"Kiran","rating":4}]},{"model_name":"iPhone 2","RAM":"3GB","ROM":"32GB","price":58000,"buyer":[{"cust_name":"Kiran","rating":4}]}]}]

Query

db.collection.aggregate({$unwind:"$models"},{$match:{"models.model_name":"iPhone 7Plus"}},{$project:{_id:0,"models.buyer.cust_name":1}},{$unwind:"$models.buyer"},{$sort:{"models.buyer.cust_name":-1}})

Result