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Explain Java MongoDB projections
While retrieving data from MongoDb collections you can select only necessary data using projections. In Java, you can project necessary data while reading the documents from a collection using the projection() method. Invoke this method on the result of find(), bypassing the names of the required filed names as −
projection(Projections.include("name", "age"));Example
Following Java examples read the documents from a collection, using projection we are displaying the values of name and age fields only.
import com.mongodb.client.FindIterable;
import com.mongodb.client.MongoCollection;
import com.mongodb.client.MongoDatabase;
import com.mongodb.client.model.Projections;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import org.bson.Document;
import com.mongodb.MongoClient;
public class ProjectionExample {
public static void main( String args[] ) {
//Creating a MongoDB client
MongoClient mongo = new MongoClient( "localhost" , 27017 );
//Connecting to the database
MongoDatabase database = mongo.getDatabase("myDatabase");
//Creating a collection object
MongoCollection<Document>collection = database.getCollection("students");
Document document1 = new Document("name", "Ram").append("age", 26).append("city", "Hyderabad");
Document document2 = new Document("name", "Robert").append("age", 27).append("city", "Vishakhapatnam");
Document document3 = new Document("name", "Rhim").append("age", 30).append("city", "Delhi");
//Inserting the created documents
List<Document> list = new ArrayList<Document>();
list.add(document1);
list.add(document2);
list.add(document3);
collection.insertMany(list);
System.out.println("Documents Inserted");
collection = database.getCollection("students");
//Retrieving the documents
FindIterable<Document> iterDoc =
collection.find().projection(Projections.include("name", "age"));
Iterator it = iterDoc.iterator();
while (it.hasNext()) {
System.out.println(it.next());
}
}
}Output
Documents Inserted
Document{{_id=5e8966533f68506911c946dc, name=Ram, age=26}}
Document{{_id=5e8966533f68506911c946dd, name=Robert, age=27}}
Document{{_id=5e8966533f68506911c946de, name=Rhim, age=30}}
Updated on: 10-Apr-2020
1K+ Views
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