@Entity
public class Example {

  @Id
  @GeneratedValue(strategy = GenerationType.AUTO)
  private UUID id;

  @Column(name = "unique_key_a")
  private UUID keyA;

  ...

  @OneToMany(fatch = FetchType.EAGER)
  @JoinColumn(name = "unique_key_a", referencedColumnName = "unique_key_a")
  private Set<Other> otherExamples;

  ...
}

java.lang.ClassCastException: demo.example.fake.entities.Example cannot be cast to java.io.Serializable
	at org.hibernate.type.CollectionType.getKeyOfOwner(CollectionType.java:411)
	at org.hibernate.engine.internal.Collections.processReachableCollection(Collections.java:165)
	at org.hibernate.event.internal.FlushVisitor.processCollection(FlushVisitor.java:46)
	at org.hibernate.event.internal.AbstractVisitor.processValue(AbstractVisitor.java:104)
	at org.hibernate.event.internal.AbstractVisitor.processValue(AbstractVisitor.java:65)
	at org.hibernate.event.internal.AbstractVisitor.processEntityPropertyValues(AbstractVisitor.java:59)
	at org.hibernate.event.internal.DefaultFlushEntityEventListener.onFlushEntity(DefaultFlushEntityEventListener.java:166)
	at org.hibernate.event.internal.AbstractFlushingEventListener.flushEntities(AbstractFlushingEventListener.java:235)
	at org.hibernate.event.internal.AbstractFlushingEventListener.flushEverythingToExecutions(AbstractFlushingEventListener.java:94)
	at org.hibernate.event.internal.DefaultFlushEventListener.onFlush(DefaultFlushEventListener.java:38)
	at org.hibernate.internal.SessionImpl.doFlush(SessionImpl.java:1454)
	at org.hibernate.internal.SessionImpl.managedFlush(SessionImpl.java:511)
	at org.hibernate.internal.SessionImpl.flushBeforeTransactionCompletion(SessionImpl.java:3283)
	at org.hibernate.internal.SessionImpl.beforeTransactionCompletion(SessionImpl.java:2479)
	at org.hibernate.engine.jdbc.internal.JdbcCoordinatorImpl.beforeTransactionCompletion(JdbcCoordinatorImpl.java:473)
	at org.hibernate.resource.transaction.backend.jdbc.internal.JdbcResourceLocalTransactionCoordinatorImpl.beforeCompletionCallback(JdbcResourceLocalTransactionCoordinatorImpl.java:178)
	at org.hibernate.resource.transaction.backend.jdbc.internal.JdbcResourceLocalTransactionCoordinatorImpl.access$300(JdbcResourceLocalTransactionCoordinatorImpl.java:39)
	at org.hibernate.resource.transaction.backend.jdbc.internal.JdbcResourceLocalTransactionCoordinatorImpl$TransactionDriverControlImpl.commit(JdbcResourceLocalTransactionCoordinatorImpl.java:271)
	at org.hibernate.engine.transaction.internal.TransactionImpl.commit(TransactionImpl.java:98)
	... 69 common frames omitted
Wrapped by: javax.persistence.RollbackException: Error while committing the transaction
	at org.hibernate.internal.ExceptionConverterImpl.convertCommitException(ExceptionConverterImpl.java:81)
	at org.hibernate.engine.transaction.internal.TransactionImpl.commit(TransactionImpl.java:101)
	at org.springframework.orm.jpa.JpaTransactionManager.doCommit(JpaTransactionManager.java:532)
	... 68 common frames omitted