如何使用JAVA在mongoDB中实现组功能

Question

数据集可在http://docs.mongodb.org/manual/tutorial/aggregation-zip-code-data-set/

中找到

现在，我正在尝试实施以下功能“按州恢复平均城市人口”：

db.zips.aggregate( { $group :
                     { _id : { state : "$state", city : "$city" },
                       pop : { $sum : "$pop" } } },
                   { $group :
                   { _id : "$_id.state",
                     avgCityPop : { $avg : "$pop" } } } )

输出：     {     “结果”：[         {             “_id”：“RI”，             “avgCityPop”：18933.28301886793         }

请帮助我JAVA，谢谢

Answer 1

试试这个：

// First $group operation
DBObject compoundId = new BasicDBObject( "state", "$state");
compoundId.append("city", "$city")
DBObject groupFields1 = new BasicDBObject( "_id", compoundId );
groupFields1.put("pop", new BasicDBObject( "$sum", "$pop"));
DBObject group1 = new BasicDBObject("$group", groupFields1);

// Second $group operation
DBObject groupFields2 = new BasicDBObject( "_id", "$_id.state");
groupFields2.put("avgCityPop", new BasicDBObject( "$avg", "$pop"));
DBObject group2 = new BasicDBObject("$group", groupFields2);

// Run aggregation
AggregationOutput output = collection.aggregate( group1, group2 );