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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}}
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