Java 和 Spring 的例子

这是使用 Spring Data 在 MongoDB 中创建和执行聚合查询的示例代码。

    try {
        MongoClient mongo = new MongoClient();
        DB db = mongo.getDB("so");
        DBCollection coll = db.getCollection("employees");

        //Equivalent to $match
        DBObject matchFields = new BasicDBObject();
        matchFields.put("dept", "Admin");
        DBObject match = new BasicDBObject("$match", matchFields);

        //Equivalent to $project
        DBObject projectFields = new BasicDBObject();
        projectFields.put("_id", 1);
        projectFields.put("name", 1);
        projectFields.put("dept", 1);
        projectFields.put("totalExp", 1);
        projectFields.put("age", 1);
        projectFields.put("languages", 1);
        DBObject project = new BasicDBObject("$project", projectFields);

        //Equivalent to $group
        DBObject groupFields = new BasicDBObject("_id", "$dept");
        groupFields.put("ageSet", new BasicDBObject("$addToSet", "$age"));
        DBObject employeeDocProjection = new BasicDBObject("$addToSet", new BasicDBObject("totalExp", "$totalExp").append("age", "$age").append("languages", "$languages").append("dept", "$dept").append("name", "$name"));
        groupFields.put("docs", employeeDocProjection);
        DBObject group = new BasicDBObject("$group", groupFields);

        //Sort results by age
        DBObject sort = new BasicDBObject("$sort", new BasicDBObject("age", 1));

        List<DBObject> aggregationList = new ArrayList<>();
        aggregationList.add(match);
        aggregationList.add(project);
        aggregationList.add(group);
        aggregationList.add(sort);
        AggregationOutput output = coll.aggregate(aggregationList);

        for (DBObject result : output.results()) {
            BasicDBList employeeList = (BasicDBList) result.get("docs");
            BasicDBObject employeeDoc = (BasicDBObject) employeeList.get(0);
            String name = employeeDoc.get("name").toString();
            System.out.println(name);
        }
    }catch (Exception ex){
        ex.printStackTrace();
    }

请参阅 JSON 格式的 resultSet 值以了解输出格式：

[{
    "_id": "Admin",
    "ageSet": [35.0, 30.0],
    "docs": [{
        "totalExp": 11.0,
        "age": 35.0,
        "languages": ["english", "hindi"],
        "dept": "Admin",
        "name": "Anna"
    }, {
        "totalExp": 10.0,
        "age": 30.0,
        "languages": ["german", "french", "english", "hindi"],
        "dept": "Admin",
        "name": "Adma"
    }]
}]

resultSet 包含每个组的一个条目，ageSet 包含该组中每个员工的年龄列表，“_ id”包含用于分组的字段的值，docs 包含每个员工的数据该组可以在我们自己的代码和 UI 中使用。