Queries in java mongodb API

Question

I am new to mongodb java api. I am trying to perform queries to my database. I ve read the database found the collections in it and I want to retrieve characteristics of users. My code:

    ServerAddress serverAdr;
    serverAdr = new ServerAddress(".. .. .., ...);

    Twitter twitter = null;
    MongoOptions options = new MongoOptions();
    options.connectionsPerHost = 10;
    MongoClient mongoClient = new MongoClient(.. ... ...", ...);
    DB db = mongoClient.getDB("trendSetters");
    System.out.println("Connect to database successfully");

    //JSONObject content = getJSONFromFile("user.json");
    Mongo mongo = null;

    Set<String> colls = db.getCollectionNames();
    mongo = new Mongo(serverAdr, options);
    mongo.setWriteConcern(WriteConcern.SAFE);
    DB db_se = mongo.getDB("iti_se");
    DBCollection incollection = db_se.getCollection("cms_users_unique");
    DBCollection outcollection = db_se.getCollection("cms_users_features");

    for (String s : colls) {
        System.out.println(s);
    }

Now I want to perform queries to retrieve from all ids the usernames for example. How is it possible to do so in java mongodb API?

EDIT: What i ve tried

 BasicDBObject query = new BasicDBObject();   
    DBCursor cursor;
    query = new BasicDBObject("followers", new BasicDBObject("$gt", 1));
    cursor = incollection.find(query);

    while(cursor.hasNext()){  
        System.out.println(cursor.next());
    }

However, it doesnot return nothing.

jmen7070 · Accepted Answer · 2014-04-25 13:03:54Z

1

You asked: "I want to perform queries to retrieve from all ids the usernames for example."

To retrieve all documents from collection you can use find() method of the DBCollection.

DBCursor cursor;
cursor = incollection.find();

After that, you can get the field value by its name for each document as below

while(cursor.hasNext()){
   System.out.println(cursor.next().get("username"));
}

If you need to add more criterias to query documents:

BasicDBObject query1;
DBCursor cursor;
query1 = new BasicDBObject("age", new BasicDBObject("$gt", 25));
cursor = incollection.find(query1);
while(cursor.hasNext()){
     System.out.println(cursor.next().get("username"));
}

answered Apr 25, 2014 at 13:03

jmen7070

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gasparms · Accepted Answer · 2014-04-25 12:18:57Z

1

DBCollections objects have method find which takes mainly a DBObject argument. For instance, if you want to find users with age > 50 this can help you.

query = new BasicDBObject("age", new BasicDBObject("$gt", 50));
cursor = incollection.find(query);

while(cursor.hasNext()){
    System.out.println(cursor.next());
}

BasicDBObject take first argument, that is a field, and second argument that is another BasicDBObject or a value. With composition of these objects you can build any query.

I recommend you take a look at MongoDB Documentation, it's pretty good.

Mongo Java Driver http://docs.mongodb.org/ecosystem/drivers/java/

See Javadoc. http://api.mongodb.org/java/current/

answered Apr 25, 2014 at 12:18

gasparms

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2 Comments

Jose Ramon Over a year ago

Basivally when I tried cursor.hasNext() it tried to change port in host WARNING: Exception executing isMaster command on /........:... java.io.IOException: couldn't connect to ...........:...

gasparms Over a year ago

Mongo class soon will be deprecated. You should use just MongoClient and from it, getDB, getCollection and work with your collection. You are mixing Mongo and MongoClient.

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