[{"key":1,"productArray":[{"requirementId":5,"product":"bat"},{"requirementId":6,"product":"Pen"},]},{"key":2},{"key":3,"productArray":[{"requirementId":1,"product":"bat"},{"requirementId":2,"product":"Pen"},{"requirementId":3,"product":"bat"},{"requirementId":4,"product":"bat"}]}]

db.collection.find({$match:{key:3}},{$project:{productArray:{"$filter":{"input":"$productArray","as":"p","cond":{$eq:["$$p.product","bat"]}}}}})