db={"product":[{"name":"some random name","price":100,"description":"description","category_id":ObjectId("5df8b7101a9a8d832cf40204")},],"category":[{"_id":ObjectId("5df8b7101a9a8d832cf40204"),"description":"Category description","name":"Electronics"}]}

db.product.aggregate([{"$group":{"_id":"$category_id","max":{"$max":"$price"}}},{"$lookup":{"from":"category","localField":"_id","foreignField":"_id","as":"categoryName",}},{"$set":{"categoryName":{"$arrayElemAt":["$categoryName.name",0]}}}])