MongoDB Java and Spring example


Example

This is an example code to create and execute the aggregate query in MongoDB using Spring Data.

    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();
    }

See the "resultSet" value in JSON format to understand the output format:

[{
    "_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"
    }]
}]

The "resultSet" contains one entry for each group, "ageSet" contains the list of age of each employee of that group, "_id" contains the value of the field that is being used for grouping and "docs" contains data of each employee of that group that can be used in our own code and UI.