org.hibernate.AssertionFailure: null id in [...] entry (don't flush the Session after an exception occurs)

Question

I set up a very basic Grails 3 web-app connected to a PostgreSQL db using jdbc. Below you can find code for Cluster domain class and the dedicated service.

It happened to call the createCluster method twice with the same slug argument:

clusterService.createCluster('Cluster 1', 'cl01')
clusterService.createCluster('Cluster 2', 'cl01')

resulting in the following exception

ERROR org.hibernate.engine.jdbc.spi.SqlExceptionHelper - ERROR: duplicate key value violates unique constraint "uk_9ig73x9wropf95ogrffcvyahk"
  Detail: Key (slug)=(cl01) already exists.
[...]
null id in myPackage.Cluster entry (don't flush the Session after an exception occurs). Stacktrace follows:
    org.hibernate.AssertionFailure: null id in myPackage.Cluster entry (don't flush the Session after an exception occurs)
    at grails.transaction.GrailsTransactionTemplate.execute(GrailsTransactionTemplate.groovy:93) ~[grails-core-3.1.1.jar:3.1.1]
    at com.usablenet.utest.AdminController.clusters(AdminController.groovy:28) ~[main/:na]
    at grails.plugin.springsecurity.web.filter.GrailsAnonymousAuthenticationFilter.doFilter(GrailsAnonymousAuthenticationFilter.groovy:53) ~[spring-security-core-3.0.3.jar:na]
    at grails.plugin.springsecurity.web.authentication.logout.MutableLogoutFilter.doFilter(MutableLogoutFilter.groovy:62) ~[spring-security-core-3.0.3.jar:na]
    at grails.plugin.springsecurity.web.SecurityRequestHolderFilter.doFilter(SecurityRequestHolderFilter.groovy:58) ~[spring-security-core-3.0.3.jar:na]
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) ~[na:1.8.0_60-ea]
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) ~[na:1.8.0_60-ea]
    at java.lang.Thread.run(Thread.java:745) [na:1.8.0_60-ea]

I assumed that the violation of the unique constraint would had been intercepted by the .validate() method, but apparently I was wrong. Ok, so I added a try/catch but it basically has no effect, the exception is not wrapped and I basically get a 500 Internal Error.

My questions are 2:

1. why isn't this exception catched as i would expect?
2. what does null id / don't flush the Session after an exception occurs error really mean? I'm not explicitly flushing the session, and of course the new record's id is null...

After checking a few threads pointing out the same issue, 2 of them hinted to set JTA to TRUE for the data source that Hibernate is using. Said that I have no clue how to do that, as far as I understood JTA is more oriented to manage multiple transactions among multiple databases, so I'd definitely say it's not my case... Am I right?

Cluster.groovy (domain class)

class Cluster {

    String name
    String slug

    static constraints = {
        name    blank: false, unique: true
        slug    blank: false, unique: true
    }

}

ClusterService.groovy

@Transactional
class ClusterService {

    public Cluster createCluster(String name, String slug, boolean flush = false) {
        Cluster cluster = new Cluster(name: name, slug: slug)
        try {
            if(cluster.validate())
                cluster.save(flush: flush)
        } catch(Exception e) {
            e.printStackTrace()
        }
        return cluster
    }
}

application.yml (db configuration)

dataSource:
    pooled: true
    driverClassName: 'org.postgresql.Driver'
    dialect: 'org.hibernate.dialect.PostgreSQLDialect'
    username: 'postgres'
    password: 'postgres'

environments:
    development:
        dataSource:
            dbCreate: create
            url: jdbc:postgresql://localhost:5432/myapp

Answer 1

While building a demo from scratch, I found out that the issue is not my code, rather in the Grails app's configuration (build.gradle), as merely changing the dependency

compile "org.grails.plugins:hibernate"

to

compile "org.grails.plugins:hibernate4"

made the problem disappear, with .validate() returning false as expected and no Error being thrown.

However I couldn't find any note about incompatibilities between Grails 3 and Hibernate 3, it would had been nice to find one :P