MapReduce mongodb：ReferenceError

Question

我在mongodb中有这个集合：

articles = {

{
        id: 102
        pagelinks: { 104, 105, 107, 108 },
        title: "page1"
},

{
        id: 104
        pagelinks: { 102, 205, 207, 108 },
        title: "page2"
},

...
}

我想要这个系列：

page_link_titles = {

{
        id: 102,
        links: { "page2", "page5", "page7", "page9"}            
},

{
        id: 104,
        links: { "page1", "page5", "page7", "page9" }            
},

...
}

我以这种方式使用mapreduce：

var map = function () {
                var output= {links:db.articles.find({id : {$in: db.articles.findOne({id:this.id}).pagelinks}}, {title: 1, _id:0})}
                    emit(this.id, output);
                };
var reduce = function(key, values) {
                var outs={ links:null}
                values.forEach(function(v){                    
                    if(outs.links ==null){
                        outs.links = v.links
                    }                     
                });
                return outs;
            };

db.articles.mapReduce（map，reduce，{out：＆＃39; page_link_titles＆＃39;}）;

我收到此错误：

    mapreduce failed: { 
        "errmsg" : "exception: ReferenceError: db is not defined near 't={links:db.articles.find({id: {$in: [d' (line2)", 
        "code" : 16722, 
        "ok" : 0
} at src/mongo/shell//collection.js:1224

有什么建议吗？

Answer 1

• 地图功能可以不引用其他集合和/或 对其他集合的操作请参阅here：等等 特别是在这里声明清楚The map function should not access the database for any reason

Answer 2

您可以尝试以下代码段：

样品采集：

db.articles.insert([
    {
        _id: 102,
        pagelinks: [ 104, 105, 107, 108 ],
        title: "page1"
    },
    {
        _id: 104,
        pagelinks: [ 102, 205, 207, 108 ],
        title: "page2"
    },
    {
        _id: 105,
        pagelinks: [ 102, 205, 207, 104 ],
        title: "page3"
    },    
    {
        _id: 107,
        pagelinks: [ 105, 205, 207, 104 ],
        title: "page3"
    }
]);

神奇：

db.articles.find().forEach( function (doc){
    var obj = { 
        "_id": doc._id 
    };
    var links = [];
    doc.pagelinks.forEach( function (x){
        var result = db.articles.findOne({ "_id": x });       
        if(result && result.title)  links.push(result.title);            
    }); 

    obj["links"] = links.filter(function (value, index, self) { 
        return self.indexOf(value) === index;
    });
    db.page_link_titles.save(obj);       
});

结果：

db.page_link_titles.find();

/* 1 */
{
    "_id" : 102,
    "links" : [ "page2", "page3" ]
}

/* 2 */
{
    "_id" : 104,
    "links" : [ "page1" ]
}

/* 3 */
{
    "_id" : 105,
    "links" : [ "page1", "page2" ]
}

/* 4 */
{
    "_id" : 107,
    "links" : [ "page3", "page2" ]
}