如何在MongoDB中对地图进行归约

Question

下面我提到了我用于地图缩小的集合。本文档的latitude属性下有两个属性，分别为longitude和location。使用这两个，我将生成一个名为distance的新属性。基于此属性，我需要对文档的顺序进行排序。

/* 1 */
{
    "_id" : ObjectId("5d7f63b270ba7629d2efcb4f"),
    "title" : "AppDividend",
    "published" : "2017-03-27",
    "authors" : [ 
        {
            "firstName" : "Krunal",
            "lastName" : "Lathiya"
        }
    ],
    "location" : {
        "name" : "Dehiwala, Field Ave, Dehiwala-Mount Lavinia, Sri Lanka",
        "latitude" : "6.849458",
        "longitude" : "79.881154"
    },
    "categories" : [ 
        "Angular", 
        "React", 
        "Vue"
    ]
}

/* 2 */
{
    "_id" : ObjectId("5d7f63ba70ba7629d2efcb50"),
    "title" : "Demonuts",
    "published" : "2016-12-10",
    "authors" : [ 
        {
            "firstName" : "Krunal",
            "lastName" : "Lathiya"
        }
    ],
    "location" : {
        "name" : "Paramount Event Spaces, 9th Ln, Sri Jayawardenepura Kotte 11222, Sri Lanka",
        "latitude" : "6.884836",
        "longitude" : "79.888131"
    },
    "categories" : [ 
        "Android", 
        "PHP"
    ]
}

/* 3 */
{
    "_id" : ObjectId("5d7f63c170ba7629d2efcb51"),
    "title" : "Scotch.io",
    "published" : "2011-12-10",
    "authors" : [ 
        {
            "firstName" : "Chris",
            "lastName" : "Sevilleja"
        }
    ],
    "location" : {
        "name" : "Royal MAS Arena, Rajakeeya Mawatha, Colombo 00700, Sri Lanka",
        "latitude" : "6.905327",
        "longitude" : "79.859914"
    },
    "categories" : [ 
        "React", 
        "Laravel", 
        "Vue", 
        "Angular", 
        "MongoDB", 
        "PHP", 
        "Android"
    ]
}

这是我的地图缩小功能。

db.blogs.mapReduce(
    function() {
        for (var index = 0; index < this.authors.length; ++index) {
            var author = this.authors[index ];
            var ds = getDistance(this.location.latitude,this.location.longitude);
            this.distance = ds;
            if(this.distance > coverage){
                emit(this._id,this);
            }
        }
    },
    function(id, values) {
        var value;
        for (var index = 0; index < values.length; ++index) {
            value = values[index];
        }
        return value;
    },
    {
        out: { inline: 1 },
        sort:{ distance: 1},
        scope:{
                lat1: 6.933762,
                lon1: 79.842961,
                coverage: 3,
                getDistance: function(lat2,lon2){
                                    var R = 6371; // Radius of the earth in km
                                    var dLat = (lat2-lat1)* (Math.PI/180);  // deg2rad below
                                    var dLon = (lon2-lon1)* (Math.PI/180); 
                                    var a = 
                                            Math.sin(dLat/2) * Math.sin(dLat/2) +
                                            Math.cos((lat1)* (Math.PI/180)) * Math.cos((lat2)* (Math.PI/180)) * 
                                            Math.sin(dLon/2) * Math.sin(dLon/2)
                                            ; 
                                    var c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1-a)); 
                                    var d = R * c; // Distance in km
                                    return d;
                               }
            }

    } 
)

当我使用distance属性排序时，结果将不会排序。如何排序结果？