Let’s use a simple example to demonstrate how the aggregation helper works. Suppose I am using MongoDB to store my employee’s travel expenses. I’ve created a collection named expenses, which store individual expenses by employee and by department. Here’s a sample document:

{ "_id" : ObjectId("503d5024ff9038cdbfcc9da4"
 
), 
"employee" : 61, 
"department" : "Sales", 
"amount" : 77, 
"type" : "airfare" }

I am auditing three departments: Sales, Engineering and Human Resources. I want to calculate each department’s average spend on airfare. I’d like to use the Aggregation Framework for the audit, so I think of the operation in terms of a pipeline:

1. Operation: Match documents where type = "airfare"; then pipe into
2. Operation: Pass only the department and the amount fields through the pipeline; then pipe into
3. Operation: Average the expense amount, grouped by department.

I will use the aggregation operators $match, $project and $group to perform each operation. Individual aggregation operations can be expressed as JSON objects, so I can think of my pipeline in JSON as:

1. First operation:

   $match: { type: "airfare"}

2. Piped into:

   $project: { department: 1, amount: 1 }

3. Piped into:

   $group: { _id: "$department", average: { $avg: "$amount" } }

   也就是說，執行以下命令：

   db.expenses.aggregate({
   $match:{type:"airfare"}$project:{_id:0,department:1,amount:1}
   $group:{_id:"$department",average:{$avg:"$amount"}}
   })

   Java 實現：

   // create our pipeline operations, first with the $matchDBObjectmatch=newBasicDBObject("$match",newBasicDBObject("type","airfare"));

   // build the $projection operationDBObjectfields=newBasicDBObject("department",1);fields.put("amount",1);fields.put("_id",0);DBObjectproject=newBasicDBObject("$project",fields);

   // Now the $group operationDBObjectgroupFields=newBasicDBObject("_id","$department");groupFields.put("average",newBasicDBObject("$avg","$amount"));DBObjectgroup=newBasicDBObject("$group",groupFields);

   // run aggregationAggregationOutputoutput=collection.aggregate(match,project,group);

   
   Aggregations are executed as database commands in MongoDB. These commands embed the results of the aggregation task in an object that also contains additional information about how the command was executed. The return value of aggregate() is an instance of the AggregationOutput class, which provides assessors to this information.
   
   

   public Iterable<DBObject> results() 
   public CommandResult getCommandResult 
   public DBObject getCommand()

   Let’s take a look at the results of my audit:

   System.out.println(output.getCommandResult());

   { "serverUsed" : "/127.0.0.1:27017" ,
    "result" : 
   [ {"_id" : "Human Resources","average": 74.91735537190083},
    {"_id" : "Sales" , "average" : 72.30275229357798},
    {"_id" : "Engineering" , "average" : 74.1} ],
    "ok" : 1.0 }