Neo4j ExecutionEngine does not return valid results

Question

Trying to use a similar example from the sample code found here

My sample function is:

void query()

{
    String nodeResult = "";
    String rows = "";
    String resultString;
    String columnsString;

    System.out.println("In query");
    // START SNIPPET: execute
    ExecutionEngine engine = new ExecutionEngine( graphDb );

    ExecutionResult result;
    try ( Transaction ignored = graphDb.beginTx() )
    {
        result = engine.execute( "start n=node(*) where n.Name =~ '.*79.*' return n, n.Name" );
        // END SNIPPET: execute
        // START SNIPPET: items
        Iterator<Node> n_column = result.columnAs( "n" );
        for ( Node node : IteratorUtil.asIterable( n_column ) )
        {
            // note: we're grabbing the name property from the node,
            // not from the n.name in this case.
            nodeResult = node + ": " + node.getProperty( "Name" );
            System.out.println("In for loop");
            System.out.println(nodeResult);

        }
        // END SNIPPET: items



        // START SNIPPET: columns
        List<String> columns = result.columns();
        // END SNIPPET: columns

        // the result is now empty, get a new one
        result = engine.execute( "start n=node(*) where n.Name =~ '.*79.*' return n, n.Name" );
        // START SNIPPET: rows
        for ( Map<String, Object> row : result )
        {
            for ( Entry<String, Object> column : row.entrySet() )
            {
                rows += column.getKey() + ": " + column.getValue() + "; ";
                System.out.println("nested");
            }
            rows += "\n";
        }
        // END SNIPPET: rows
        resultString = engine.execute( "start n=node(*) where n.Name =~ '.*79.*' return n.Name"  ).dumpToString();
        columnsString = columns.toString();
        System.out.println(rows);
        System.out.println(resultString);
        System.out.println(columnsString);

        System.out.println("leaving");

    }

}

When I run this in the web console I get many results (as there are multiple nodes that have an attribute of Name that contains the pattern 79. Yet running this code returns no results. The debug print statements 'in loop' and 'nested' never print either. Thus this must mean there are not results found in the Iterator, yet that doesn't make sense.

And yes, I already checked and made sure that the graphDb variable is the same as the path for the web console. I have other code earlier that uses the same variable to write to the database.

EDIT - More info If I place the contents of query in the same function that creates my data, I get the correct results. If I run the query by itself it returns nothing. It's almost as the query works only in the instance where I add the data and not if I come back to the database cold in a separate instance.

EDIT2 -

Here is a snippet of code that shows the bigger context of how it is being called and sharing the same DBHandle

package ContextEngine;

import ContextEngine.NeoHandle;
import java.util.LinkedList;


/*
 * Class to handle streaming data from any coded source
 */
public class Streamer {

    private NeoHandle myHandle;

    private String contextType;

    Streamer()
    {

    }


    public void openStream(String contextType)
    {

        myHandle = new NeoHandle();
        myHandle.createDb();
    }

    public void streamInput(String dataLine)
    {
        Context context = new Context();
        /*
         * get database instance
         * write to database
         * check for errors
         * report errors & success
         */
        System.out.println(dataLine);

        //apply rules to data (make ContextRules do this, send type and string of data)
        ContextRules contextRules = new ContextRules();
        context = contextRules.processContextRules("Calls", dataLine);

        //write data (using linked list from contextRules)
        NeoProcessor processor = new NeoProcessor(myHandle);
        processor.processContextData(context);
    }

    public void runQuery()
    {
        NeoProcessor processor = new NeoProcessor(myHandle);
        processor.query();

    }

    public void closeStream()
    {
        /*
         * close database instance
         */
        myHandle.shutDown();
    }

}

Now, if I call streamInput AND query in in the same instance (parent calls) the query returns results. If I only call query and do not enter ANY data in that instance (yet web console shows data for same query) I get nothing. Why would I have to create the Nodes and enter them into the database at runtime just to return a valid query. Shouldn't I ALWAYS get the same results with such a query?

Answer 1

You mention that you are using the Neo4j Browser, which comes with Neo4j. However, the example you posted is for Neo4j Embedded, which is the in-process version of Neo4j. Are you sure you are talking to the same database when you try your query in the Browser?

In order to talk to Neo4j Server from Java, I'd recommend looking at the Neo4j JDBC driver, which has good support for connecting to the Neo4j server from Java.

http://www.neo4j.org/develop/tools/jdbc

You can set up a simple connection by adding the Neo4j JDBC jar to your classpath, available here: https://github.com/neo4j-contrib/neo4j-jdbc/releases Then just use Neo4j as any JDBC driver:

Connection conn = DriverManager.getConnection("jdbc:neo4j://localhost:7474/");
ResultSet rs = conn.executeQuery("start n=node({id}) return id(n) as id", map("id", id));

while(rs.next()) {
    System.out.println(rs.getLong("id"));
}

Refer to the JDBC documentation for more advanced usage.

To answer your question on why the data is not durably stored, it may be one of many reasons. I would attempt to incrementally scale back the complexity of the code to try and locate the culprit. For instance, until you've found your problem, do these one at a time:

1. Instead of looping through the result, print it using System.out.println(result.dumpToString());

2. Instead of the regex query, try just MATCH (n) RETURN n, to return all data in the database

3. Make sure the data you are seeing in the browser is not "old" data inserted earlier on, but really is an insert from your latest run of the Java program. You can verify this by deleting the data via the browser before running the Java program using MATCH (n) OPTIONAL MATCH (n)-[r]->() DELETE n,r;

4. Make sure you are actually working against the same database directories. You can verify this by leaving the server running. If you can still start your java program, unless your Java program is using the Neo4j REST Bindings, you are not using the same directory. Two Neo4j databases cannot run against the same database directory simultaneously.