This affects Hibernate ORM 6 and 5.6.

This is only relevant with bytecode enhancement enabled to get [lazy basic properties](https://docs.jboss.org/hibernate/orm/current/userguide/html_single/Hibernate_User_Guide.html#BytecodeEnhancement-lazy-loading).

Updating such lazy properties does not seem to work correctly:

• When properties are assigned to fetch groups, there is only one problem: setting a lazy property to null is ignored, the entity is not updated.
• When properties are not assigned to fetch groups:
  • setting a lazy property to null is ignored, the entity is not updated.
  • setting a lazy property to a non-null value is ignored, the entity is not updated.
  • changing only a single lazy property on an entity that has only lazy properties will lead to an exception:

    java.lang.IllegalArgumentException: sql must be non-null.
    
    	at org.hibernate.engine.jdbc.batch.internal.AbstractBatchImpl.getBatchStatement(AbstractBatchImpl.java:127)
    	at org.hibernate.persister.entity.AbstractEntityPersister.update(AbstractEntityPersister.java:3544)
    	at org.hibernate.persister.entity.AbstractEntityPersister.updateOrInsert(AbstractEntityPersister.java:3470)
    	at org.hibernate.persister.entity.AbstractEntityPersister.update(AbstractEntityPersister.java:3887)
    	at org.hibernate.action.internal.EntityUpdateAction.execute(EntityUpdateAction.java:136)
    	at org.hibernate.engine.spi.ActionQueue.executeActions(ActionQueue.java:615)
    	at org.hibernate.engine.spi.ActionQueue.lambda$executeActions$1(ActionQueue.java:486)
    	at java.base/java.util.LinkedHashMap.forEach(LinkedHashMap.java:684)
    	at org.hibernate.engine.spi.ActionQueue.executeActions(ActionQueue.java:483)
    	at org.hibernate.event.internal.AbstractFlushingEventListener.performExecutions(AbstractFlushingEventListener.java:329)
    	at org.hibernate.event.internal.DefaultFlushEventListener.onFlush(DefaultFlushEventListener.java:39)
    	at org.hibernate.event.service.internal.EventListenerGroupImpl.fireEventOnEachListener(EventListenerGroupImpl.java:107)
    	at org.hibernate.internal.SessionImpl.doFlush(SessionImpl.java:1422)
    	at org.hibernate.internal.SessionImpl.managedFlush(SessionImpl.java:476)
    	at org.hibernate.internal.SessionImpl.flushBeforeTransactionCompletion(SessionImpl.java:2233)
    	at org.hibernate.internal.SessionImpl.beforeTransactionCompletion(SessionImpl.java:1929)
    	at org.hibernate.engine.jdbc.internal.JdbcCoordinatorImpl.beforeTransactionCompletion(JdbcCoordinatorImpl.java:439)
    	at org.hibernate.resource.transaction.backend.jdbc.internal.JdbcResourceLocalTransactionCoordinatorImpl.beforeCompletionCallback(JdbcResourceLocalTransactionCoordinatorImpl.java:183)
    	at org.hibernate.resource.transaction.backend.jdbc.internal.JdbcResourceLocalTransactionCoordinatorImpl$TransactionDriverControlImpl.commit(JdbcResourceLocalTransactionCoordinatorImpl.java:281)
    	at org.hibernate.engine.transaction.internal.TransactionImpl.commit(TransactionImpl.java:101)
    	at org.hibernate.testing.transaction.TransactionUtil.doInHibernate(TransactionUtil.java:372)
    	at org.hibernate.orm.test.bytecode.enhancement.lazy.basic.OnlyLazyBasicUpdateTest.updateSomeLazyProperty(OnlyLazyBasicUpdateTest.java:57)
    	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    	at java.base/java.lang.reflect.Method.invoke(Method.java:566)
    	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:59)
    	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
    	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:56)
    	at org.hibernate.testing.junit4.ExtendedFrameworkMethod.invokeExplosively(ExtendedFrameworkMethod.java:45)
    	at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
    	at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
    	at org.junit.internal.runners.statements.RunAfters.evaluate(RunAfters.java:27)
    	at org.junit.internal.runners.statements.FailOnTimeout$CallableStatement.call(FailOnTimeout.java:299)
    	at org.junit.internal.runners.statements.FailOnTimeout$CallableStatement.call(FailOnTimeout.java:293)
    	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
    	at java.base/java.lang.Thread.run(Thread.java:829)

I will add a PR with a reproducer here.