Troubleshoot the Map Function对Map函数进行故障排除

Verify Key and Value Pairs验证键和值对

To verify the key and value pairs emitted by the map function, write your own emit function.要验证map函数发出的键和值对，请编写自己的emit函数。

Consider a collection orders that contains documents of the following prototype:考虑包含以下原型文档的集合orders：

{
     _id: ObjectId("50a8240b927d5d8b5891743c"),
     cust_id: "abc123",
     ord_date: new Date("Oct 04, 2012"),
     status: 'A',
     price: 250,
     items: [ { sku: "mmm", qty: 5, price: 2.5 },
              { sku: "nnn", qty: 5, price: 2.5 } ]
}

1. Define the map function that maps the price to the cust_id for each document and emits the cust_id and price pair:定义映射函数，将每个文档的price映射到cust_id，并发出cust_id和price对：

   var map = function() {
       emit(this.cust_id, this.price);
   };

2. Define the emit function to print the key and value:定义emit函数以打印键和值：

   var emit = function(key, value) {
       print("emit");
       print("key: " + key + "  value: " + tojson(value));
   }

3. Invoke the map function with a single document from the orders collection:使用orders集合中的单个文档调用map函数：

   var myDoc = db.orders.findOne( { _id: ObjectId("50a8240b927d5d8b5891743c") } );
   map.apply(myDoc);

4. Verify the key and value pair is as you expected.验证密钥和值对是否符合预期。

   emit
   key: abc123 value:250

5. Invoke the map function with multiple documents from the orders collection:使用orders集合中的多个文档调用map函数：

   var myCursor = db.orders.find( { cust_id: "abc123" } );
   while (myCursor.hasNext()) {
       var doc = myCursor.next();
       print ("document _id= " + tojson(doc._id));
       map.apply(doc);
       print();
   }

6. Verify the key and value pairs are as you expected.验证密钥和值对是否符合预期。