DSE Graph with Java Driver, how to add edges

I want to build a graph completely with the Datastax Java Driver. I managed to insert vertices, but I have no clue how to add edges to existing vertices.

When I run the following code

session.executeGraph("parent = g.V().has('businessId','sys-1').next()");
session.executeGraph("child = g.V().has('businessId','sys-2').next()");
session.executeGraph("parent.addEdge('consistsOf', child)");

I get an exception

Exception in thread "main" com.datastax.driver.core.exceptions.InvalidQueryException: No such property: parent for class: Script285
    at com.datastax.driver.core.exceptions.InvalidQueryException.copy(InvalidQueryException.java:50)
    at com.datastax.driver.dse.DriverThrowables.propagateCause(DriverThrowables.java:29)
    at com.datastax.driver.dse.DefaultDseSession.executeGraph(DefaultDseSession.java:77)
    at com.datastax.driver.dse.DefaultDseSession.executeGraph(DefaultDseSession.java:64)
    at de.pratho.valpro.tools.Main.main(Main.java:41)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:497)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)
Caused by: com.datastax.driver.core.exceptions.InvalidQueryException: No such property: parent for class: Script285
    at com.datastax.driver.core.Responses$Error.asException(Responses.java:136)
    at com.datastax.driver.core.DefaultResultSetFuture.onSet(DefaultResultSetFuture.java:179)
    at com.datastax.driver.core.RequestHandler.setFinalResult(RequestHandler.java:173)
    at com.datastax.driver.core.RequestHandler.access$2500(RequestHandler.java:43)
    at com.datastax.driver.core.RequestHandler$SpeculativeExecution.setFinalResult(RequestHandler.java:788)
    at com.datastax.driver.core.RequestHandler$SpeculativeExecution.onSet(RequestHandler.java:607)
    at com.datastax.driver.core.Connection$Dispatcher.channelRead0(Connection.java:1012)
    at com.datastax.driver.core.Connection$Dispatcher.channelRead0(Connection.java:935)
    at io.netty.channel.SimpleChannelInboundHandler.channelRead(SimpleChannelInboundHandler.java:105)
    at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:318)
    at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:304)
    at io.netty.handler.timeout.IdleStateHandler.channelRead(IdleStateHandler.java:266)
    at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:318)
    at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:304)
    at io.netty.handler.codec.MessageToMessageDecoder.channelRead(MessageToMessageDecoder.java:103)
    at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:318)
    at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:304)
    at io.netty.handler.codec.ByteToMessageDecoder.fireChannelRead(ByteToMessageDecoder.java:276)
    at io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:263)
    at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:318)
    at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:304)
    at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:846)
    at io.netty.channel.nio.AbstractNioByteChannel$NioByteUnsafe.read(AbstractNioByteChannel.java:131)
    at io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:511)
    at io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:468)
    at io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:382)
    at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:354)
    at io.netty.util.concurrent.SingleThreadEventExecutor$2.run(SingleThreadEventExecutor.java:112)
    at java.lang.Thread.run(Thread.java:745)

When I run the gremlin statements in the gremlin_console_window it is working fine. So I think variables like parent and child are not working within a Java DseSession?

Unfortunately, I was not able to find much information about how to work with the Java Driver properly.

It looks like you have to create it within the context of the same script, i.e:

DseCluster dseCluster = DseCluster.builder()
            .addContactPoint("127.0.0.1")
            .withGraphOptions(new GraphOptions().setGraphName("demo"))
            .build();
DseSession dseSession = dseCluster.newSession();
SimpleGraphStatement s = new SimpleGraphStatement(
            "def v1 = g.V(id1).next()\n" + 
            "def v2 = g.V(id2).next()\n" +
            "v1.addEdge('relates', v2)");
dseSession.executeGraph(s);

I think the reason for this is that these commands are just interpreted as independent gremlin queries.

I believe this set of documentation may be helpful to you.