Caused by: org.postgresql.util.PSQLException: ERROR: column "testsibling" is of type uuid but expression is of type character varying
  Hint: You will need to rewrite or cast the expression.
  Position: 62

public class UUIDConverter implements org.eclipse.persistence.mappings.converters.Converter
{
  private static final Logger LOG = Logger.getLogger(UUIDConverter.class.getName());

  @Override
  public UUID convertObjectValueToDataValue(Object objectValue, Session session)
  {
    return (UUID) objectValue;
  }

  @Override
  public UUID convertDataValueToObjectValue(Object dataValue, Session session)
  {
    return (UUID) dataValue;
  }

  @Override
  public boolean isMutable()
  {
    return false;
  }

  @Override
  public void initialize(DatabaseMapping mapping, Session session)
  {
    DatabaseField field = mapping.getField();
    field.setSqlType(Types.OTHER);
    field.setTypeName("java.util.UUID");
    field.setColumnDefinition("UUID");


    if (LOG.isLoggable(Level.FINE))
      LOG.log(Level.FINE, "Setting local field " + field + " to be a UUID");

    // Find the bi-directional references other objects have to this field, and update them to be uuid too
    for (DatabaseMapping m : mapping.getDescriptor().getMappings())
    {
      if (m instanceof OneToOneMapping)
      {
        OneToOneMapping oneToOneMapping = (OneToOneMapping) m;
        
        DatabaseField relationshipField = oneToOneMapping.getSourceToTargetKeyFields().get(field);
        if (relationshipField != null)
        {
          relationshipField.setSqlType(Types.OTHER);
          relationshipField.setColumnDefinition("UUID");
          relationshipField.setTypeName("java.util.UUID");
          if (LOG.isLoggable(Level.FINE))
            LOG.log(Level.FINE, "Setting foreign key field " + relationshipField + " to be a UUID");
        }
      }
    }
  }
}

@Entity
@NamedQuery(name="Test.findByField1", query="SELECT T from Test T where T.field1 = :field1")
public class Test
{

  @Id
  @Converter(converterClass=UUIDConverter.class, name="test_uuid")
  @Convert("test_uuid")
  @Column(name="test")
  private UUID test = UUID.randomUUID();
  
  private String field1;

  @OneToOne(optional=true)
  @JoinColumn(name="testsibling")
  private TestSibling testSibling;

}

@Entity
public class TestSibling
{

  @Id
  @Converter(converterClass=UUIDConverter.class, name="testsibling_uuid")
  @Convert("testsibling_uuid")
  @Column(name="testsibling")
  private UUID testSibling = UUID.randomUUID();
  
  private String field2;

//  @OneToOne(mappedBy="testSibling")
//  private Test test;

 }

  @Schedule(second="*/5", minute="*", hour="*")
  @Timeout
  public void start()
  {
    Test test = new Test();
    test.setField1(Integer.toString(new Random().nextInt()));
    em.persist(test);
    
    LOG.info("Victory");
  }

    drop cast if exists (character varying as uuid);
    create or replace function uuid(_text character varying) returns uuid language sql as 'select uuid_in(_text::cstring)';
    create cast (character varying as uuid) with function uuid(character varying) as assignment;

public class UUIDConverter implements org.eclipse.persistence.mappings.converters.Converter
{
  private static final long serialVersionUID = 1L;
  
  @Override
  public UUID convertObjectValueToDataValue(Object objectValue, Session session)
  {
    return (UUID) objectValue;
  }

  @Override
  public UUID convertDataValueToObjectValue(Object dataValue, Session session)
  {
    return (UUID) dataValue;
  }

  @Override
  public boolean isMutable()
  {
    return false;
  }

  @Override
  public void initialize(DatabaseMapping mapping, Session session)
  {
    DatabaseField field = mapping.getField();
    field.setSqlType(Types.OTHER);
    field.setTypeName("java.util.UUID");
    field.setColumnDefinition("UUID");
  }