MongoDB hello: Shows how to update a field with the value of another field by using either eval (nolock, for mongodb >= 1.8) or map/reduce (mongodb 1.6).

MongoDBUpdateFieldByAnotherField.java

package hello.mongodb;

import java.net.UnknownHostException;

import com.mongodb.BasicDBObject;
import com.mongodb.BasicDBObjectBuilder;
import com.mongodb.DB;
import com.mongodb.DBCollection;
import com.mongodb.DBObject;
import com.mongodb.MapReduceOutput;
import com.mongodb.Mongo;
import com.mongodb.MongoException;

public class MongoDBUpdateFieldByAnotherField {

    public static void main( final String[] args ) throws UnknownHostException, MongoException {
        final Mongo mongo = new Mongo();

        final DB db = mongo.getDB( "helloMR" );
        final DBCollection things = db.getCollection( "things" );
        things.remove( new BasicDBObject() ); // just clean up

        things.insert( new BasicDBObject( "currentVersion", Integer.valueOf( 1 ) ).append( "newVersion", Integer.valueOf( 2 ) ) );
        things.insert( new BasicDBObject( "currentVersion", Integer.valueOf( 2 ) ).append( "newVersion", Integer.valueOf( 2 ) ) );
        System.out.println( "Have items with version 2: " + things.find( new BasicDBObject( "currentVersion", Integer.valueOf( 2 ) ) ).count() );
        
        final int numUpdated;
        final String version = (String) db.command( "serverStatus" ).get( "version" );
        if ( version.startsWith( "1.8" ) ) { // a very primitive check on the version...
            System.out.println( "Running map/reduce in 1.8 mode." );
            numUpdated = updateCurrentVersionViaNewNonBlockingEval( things );
        }
        else {
            System.out.println( "Running map/reduce in 1.6 mode (actual server version: " + version + ")." );
            numUpdated = updateCurrentVersionViaOldMapReduce( things );
        }

        System.out.println( "Updated " + numUpdated + " items." );
        System.out.println( "Have items with version 2: " + things.find( new BasicDBObject( "currentVersion", Integer.valueOf( 2 ) ) ).count() );

    }

    /**
     * Sets the currentVersion to newVersion if currentVersion < newVersion using eval without locking (since 1.8).
     * @return the number of updated things.
     */
    private static int updateCurrentVersionViaNewNonBlockingEval( final DBCollection things ) throws MongoException {
        final String code = "var i = 0;" +
                "db.things.find({ '$where' : 'this.currentVersion < this.newVersion' })" +
                ".forEach( function(doc) {" +
                        " doc.currentVersion = doc.newVersion;" +
                        " db.things.save(doc);" +
                        " i += 1; } );" +
                "return i;";
        final CommandResult commandResult = things.getDB().command( BasicDBObjectBuilder.start()
                .add( "$eval" , code  )
                .add( "nolock" , Boolean.TRUE )
                .get() );
        commandResult.throwOnError();
        return commandResult.getInt( "retval" );
    }

    /**
     * Sets the currentVersion to newVersion if currentVersion &lt; newVersion using map/reduce (mongodb 1.6).
     * @return the number of updated things.
     */
    @SuppressWarnings( { "boxing" } )
    private static int updateCurrentVersion( final DBCollection things ) throws MongoException {

        final String mapFunction = "function () {" +
                " if (this.currentVersion < this.newVersion) {" +
                "   emit(this._id, this);" +
                " } }";

        final String reduceFunction = "function (key, values) { return values[0]; }";

        final String finalizeFunction = "function (key, value) { " +
                "db.things.update(" +
                        "{ _id : value._id }, " +
                        "{ $set : { 'currentVersion' : value.newVersion } } ); }";

        final BasicDBObjectBuilder b = BasicDBObjectBuilder.start()
            .add("mapreduce", "things")
            .add("map", mapFunction )
            .add("reduce", reduceFunction )
            .add("finalize", finalizeFunction );

        final MapReduceOutput output = things.mapReduce( b.get() );

        return (Integer) ((DBObject)output.getRaw().get( "counts" )).get( "emit" );

    }

}