java spring-boot neo4j spring-data-neo4j neo4j-ogm

What is the cause of Cannot merge node using null property value for uuid in Spring Data Neo4j starter and OGM SessionFactory?

Apparently this is a common exception but, all the questions asked here seem to be related to CSV hence, why this question.

I have a domain object that make use of uuid as unique id and is declared as follow

@NodeEntity(label = "TNODE")
public class TestNode {
    @Id @GeneratedValue(strategy = UuidStrategy.class) 
    @Convert(UuidStringConverter.class)
    private UUID uuid;

    private String name; 

    @Relationship(type = "TEST_REL_IS", direction = Relationship.OUTGOING) private TestNodeTarget testTarget;

    public TestNode() {}
    public TestNode(String name, TestNodeTarget target) {
        this.name = name; 
        this.testTarget = target;
    }

    //getters and setters
}

And the TestNodeTarget is as follow

@NodeEntity(label = "TNODE_TARGET")
public class TestNodeTarget {
    private Long id; 
    private String name;

    public TestNodeTarget() {}
    public TestNodeTarget(String name) {
        this.name = name; 
    }

    //getters and setters
}

I am using Spring Boot 2.0.3.RELEASE with sprng-boot-data-neo4j starter which pulls down neo4j-ogm-core-3.1.0, neo4j-ogm-bolt-driver-3.1.0, neo4j-ogm-api-3.1.0 and The mysterious neo4j-java-driver-1.5.

I am using the Neo4j OGM SessionFactory to build my own generic DAO layer instead of Spring Data Repository that restricts me from

Single Repository for all domains (i.e. loadById(classType, id))
Run custom cypher queries
Some other reasons I don't remember

Issue

I am able to perform CREATE, READ and DELETE operations. The PUT operations with following assumptions

Previously created tn:TestNode, t1:TestNodeTarget and t2:TestNodeTarget
The tn node has a relationship with t1 node
Retrieve tn node, t1 & t2 nodes
Change the tn node relationship to t2 from t1
Call session.save

The exception is

Caused by: org.neo4j.driver.v1.exceptions.ClientException: Cannot merge node using null property value for uuid
    at org.neo4j.driver.internal.util.ErrorUtil.newNeo4jError(ErrorUtil.java:62)
    at org.neo4j.driver.internal.async.inbound.InboundMessageDispatcher.handleFailureMessage(InboundMessageDispatcher.java:137)
    at org.neo4j.driver.internal.messaging.PackStreamMessageFormatV1$Reader.unpackFailureMessage(PackStreamMessageFormatV1.java:432)
    at org.neo4j.driver.internal.messaging.PackStreamMessageFormatV1$Reader.read(PackStreamMessageFormatV1.java:396)
    at org.neo4j.driver.internal.async.inbound.InboundMessageHandler.channelRead0(InboundMessageHandler.java:83)
    at org.neo4j.driver.internal.async.inbound.InboundMessageHandler.channelRead0(InboundMessageHandler.java:35)
    at org.neo4j.driver.internal.shaded.io.netty.channel.SimpleChannelInboundHandler.channelRead(SimpleChannelInboundHandler.java:105)
    at org.neo4j.driver.internal.shaded.io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
    at org.neo4j.driver.internal.shaded.io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
    at org.neo4j.driver.internal.shaded.io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:340)
    at org.neo4j.driver.internal.shaded.io.netty.handler.codec.ByteToMessageDecoder.fireChannelRead(ByteToMessageDecoder.java:310)
    at org.neo4j.driver.internal.shaded.io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:284)
    at org.neo4j.driver.internal.async.inbound.MessageDecoder.channelRead(MessageDecoder.java:40)
    at org.neo4j.driver.internal.shaded.io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
    at org.neo4j.driver.internal.shaded.io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
    at org.neo4j.driver.internal.shaded.io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:340)
    at org.neo4j.driver.internal.shaded.io.netty.handler.codec.ByteToMessageDecoder.fireChannelRead(ByteToMessageDecoder.java:310)
    at org.neo4j.driver.internal.shaded.io.netty.handler.codec.ByteToMessageDecoder.fireChannelRead(ByteToMessageDecoder.java:297)
    at org.neo4j.driver.internal.shaded.io.netty.handler.codec.ByteToMessageDecoder.callDecode(ByteToMessageDecoder.java:413)
    at org.neo4j.driver.internal.shaded.io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:265)
    at org.neo4j.driver.internal.shaded.io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
    at org.neo4j.driver.internal.shaded.io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
    at org.neo4j.driver.internal.shaded.io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:340)
    at org.neo4j.driver.internal.shaded.io.netty.handler.ssl.SslHandler.unwrap(SslHandler.java:1336)
    at org.neo4j.driver.internal.shaded.io.netty.handler.ssl.SslHandler.decodeJdkCompatible(SslHandler.java:1127)
    at org.neo4j.driver.internal.shaded.io.netty.handler.ssl.SslHandler.decode(SslHandler.java:1162)
    at org.neo4j.driver.internal.shaded.io.netty.handler.codec.ByteToMessageDecoder.decodeRemovalReentryProtection(ByteToMessageDecoder.java:489)
    at org.neo4j.driver.internal.shaded.io.netty.handler.codec.ByteToMessageDecoder.callDecode(ByteToMessageDecoder.java:428)
    at org.neo4j.driver.internal.shaded.io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:265)
    at org.neo4j.driver.internal.shaded.io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
    at org.neo4j.driver.internal.shaded.io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
    at org.neo4j.driver.internal.shaded.io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:340)
    at org.neo4j.driver.internal.shaded.io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1359)
    at org.neo4j.driver.internal.shaded.io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
    at org.neo4j.driver.internal.shaded.io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
    at org.neo4j.driver.internal.shaded.io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:935)
    at org.neo4j.driver.internal.shaded.io.netty.channel.nio.AbstractNioByteChannel$NioByteUnsafe.read(AbstractNioByteChannel.java:134)
    at org.neo4j.driver.internal.shaded.io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:645)
    at org.neo4j.driver.internal.shaded.io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:580)
    at org.neo4j.driver.internal.shaded.io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:497)
    at org.neo4j.driver.internal.shaded.io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:459)
    at org.neo4j.driver.internal.shaded.io.netty.util.concurrent.SingleThreadEventExecutor$5.run(SingleThreadEventExecutor.java:858)
    at org.neo4j.driver.internal.shaded.io.netty.util.concurrent.DefaultThreadFactory$DefaultRunnableDecorator.run(DefaultThreadFactory.java:138)

Questions

What is the root cause of the error Caused by: org.neo4j.driver.v1.exceptions.ClientException: Cannot merge node using null property value for uuid message? The node uuid value is not null.
How do bolt-driver and neo4j-java-driver communicate with each other? I was surprised to see neo4j-java-driver.
Since I am using Neo4j OGM in favour of Spring Data's Repository then can I ditch the spring-boot-starter-data-neo4j? If yes, what happens to the @Transactional and transaction management?

Solution

TL;DR

Using a UUID field annotated with @Id is not a replacement to private Long id field (or is it?). Add the id field to the class and the exception disappears.

I couldn't wait and decided to debug this issue myself and share my findings

Transaction

At first, the PUT operation worked (without the private Long id field) without exception but failed to delete the old relationship and based on this Github issue I wrapped the operations in @Transactional and I got the exception. Some progress.

In ORM context, the above would work fine. In other words, I can retrieve the entity from @Transactional annotated service layer, make necessary changes in a non-transactional layer and then persist the entity using another service layer method that is annotated with @Transactional. For OGM you gotta do it all in same transaction layer. Hum!

The exception

As shown in the question above, the TestNode entity makes use of UUID as follow

@Id @GeneratedValue(strategy = UuidStrategy.class) 
@Convert(UuidStringConverter.class)
private UUID uuid;

I had to update the TestNode class to include a

private Long id;

And after this the PUT operation works as expected and the old relationship gets removed.

Confusions

Why lack of private Long id; only cause exception during update? Why not when you retrieve, delete, or create the entity?