JavaAppEngine数据存储:如何查询对象继承类的字段?

见1.2.2。我这样定义一个类产品:

@PersistenceCapable(identityType = IdentityType.APPLICATION, table="Products")
public class Product {

 public Product(String title) {
  super();
  this.title = title;
 }

 public String getTitle() {
  return title;
 }

 @Persistent
 String title;

 @PrimaryKey
 @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
 private Key id;
}

我这样定义派生类书籍:

@PersistenceCapable(identityType = IdentityType.APPLICATION, table="Products")
public class Book extends Product {

 public Book(String author, String title) {
  super(title);
  this.author = author;
 }

 public String getAuthor() {
  return author;
 }

 @Persistent
 String author;
}

然后我做了一个这样的新物体:

persistenceManager pm=pmf.get().getPersistenceManager(); pm.makepersistent(新书(“George Orwell”,“1984”));

我可以使用如下查询来查询这个新对象:

query query=pm.newquery(“select from”+book.class.getname()+“where author==param”); query.declareparameters(“string param”); list results=(list)query.execute(“george orwell”);

这将返回对象,因为我正在查询Book上定义的字段“author”。

但是这不起作用:

query query=pm.newquery(“select from”+book.class.getname()+“where title==param”); query.declareparameters(“string param”); list results=(list)query.execute(“1984”);

它抛出一个异常,说明没有字段“title”,即使在派生类产品上定义了这个字段。

javax.jdo.JDOUserException: Field "title" does not exist in com.example.Book or is not persistent
NestedThrowables:
org.datanucleus.store.exceptions.NoSuchPersistentFieldException: Field "title" does not exist in com.example.Book or is not persistent

似乎继承类中的字段在数据存储查询中不可用。

事实上,这在语法上是可能的,还是在注释上?