从mongoldb mapReduce访问数据库变量时出错

Hi'我正在编写一个使用MongoDB数据库的应用程序。我有一个用户集合，用于存储所有用户数据。文件具有以下形式：

{
    "_id" : ObjectId("542e67e07f724fc2af28ba75"),
    "id" : "",
    "email" : "luigi@gmail.com",
    "tags" : [
        {
            "tag" : "Paper Goods:Liners - Baking Cups",
            "weight" : 2,
            "lastInsert" : 1412327492874
        },
        {
            "tag" : "Vegetable:Carrots - Jumbo",
            "weight" : 4,
            "lastInsert" : 1412597883569
        },
        {
            "tag" : "Paper Goods:Lialberto- Baking Cups",
            "weight" : 1,
            "lastInsert" : 1412327548205
        },
        {
            "tag" : "Fish:Swordfish Loin Portions",
            "weight" : 3,
            "lastInsert" : 1412597939124
        },
        {
            "tag" : "Vegetable:Carrots - alberto@gmail.com",
            "weight" : 2,
            "lastInsert" : 1412597939124
        }
    ]
}

现在我创建了一个recommendation.tagsMatch集合，用于存储两个标签之间的相似性。

收集的文件具有以下形式：

{
    "_id" : "Fish:Swordfish Loin Portions-Paper Goods:Lialberto- Baking Cups",
    "value" : {
        "tag1" : "Fish:Swordfish Loin Portions",
        "tag2" : "Paper Goods:Lialberto- Baking Cups",
        "sum1" : 3,
        "sum2" : 1,
        "sumQ1" : 9,
        "sumQ2" : 1,
        "sumProd" : 3,
        "count" : 2
    }
}

现在我正在编写一个从recommendation.users文档到recommendation.tagsMatch文档的mapReduce。

如果是第一次映射文档，则value.count字段必须为0，否则value.count必须是相应recommendation.tagsMatch文档中的旧值。

我已经实现了我的mapReduce功能如下：

var f1 = function() {
            var rows = this.tags;    
            if (rows != undefined) {
                rows = rows.sort(function(a, b){return a.tag > b.tag}) 
                rows.forEach( function(rowThis) {
                    if (rows != undefined) {
                        /** prendo solo i maggiori di quello che sto confrontando */
                        var toCompare = rows.filter(function(e){return rowThis.tag  < e.tag;})
                        toCompare.forEach( function(rowThat) {
                        var key = rowThis.tag + "-" + rowThat.tag;
                var documentValue = db.recommendation.tagsMatch.find({_id: key}).one()
                    var countValue = 0
                if(documentValue.value.count != undefined) 
                        countValue = documentValue.value.count                          
                            var value = {
                                tag1: rowThis.tag,
                                tag2: rowThat.tag,
                                sum1: rowThis.weight,
                                sum2: rowThat.weight,
                                sumQ1: Math.pow(rowThis.weight, 2),
                                sumQ2: Math.pow(rowThat.weight, 2),
                                sumProd: rowThis.weight * rowThat.weight,
                                count: countValue
                            }
                            emit(key, value);
                        });
                    }
                });
            }
        };

var r1 = function(key, values) {
            var reducedObject = {
                tag1: "",
                tag2: "",
                sum1: 0,
                sum2: 0,
                sumQ1: 0,
                sumQ2: 0,
                sumProd: 0,
                count: 0
            }
            values.forEach( function(value) {
                reducedObject.tag1 = value.tag1;
                reducedObject.tag2 = value.tag2;
                reducedObject.sum1 += value.sum1;
                reducedObject.sum2 += value.sum2;
                reducedObject.sumQ1 += value.sumQ1;
                reducedObject.sumQ2 += value.sumQ2;
                reducedObject.sumProd += value.sumProd;
                reducedObject.count += value.count;
            });
            return reducedObject; 
        };

我将te mapReduce命令称为：

db.recommendation.users.mapReduce(f2, r1,{query: {}, "out": "recommendation.tagsMatch"}).

但是控制台给了我以下错误：

2014-10-27T17:01:28.923+0100 map reduce failed:{
    "errmsg" : "exception: ReferenceError: db is not defined near 'ntValue = db.recommendation.tagsMatch.fin'  (line 11)",
    "code" : 16722,
    "ok" : 0
}

怎么了？？我该怎么解决？？