通过密钥查询数据存储库时空了 [英] Got Null when querying datastore by key

问题描述

我有两种模式，书和章，采用一对多的关系。我手动创建Book和Chapter的键。坚持下来，我创建一个书对象，然后添加一个章节的实例，然后坚持书。这很好，因为我在数据存储中看到它们。现在，当我试图通过键从数据存储中获取一章时，我得到一个空对象。

以下是键在数据存储中的外观：

 在Book：name / id = 123章节= [Book（123）/ Chapter（abc）] 
章节：name / id = abc 
  

我创建了我的密钥，用于创建和提取对象，使用

  Key key = KeyFactory.createKey（Chapter.class.getSimpleName（），chapterId）; 
  

我的代码是这样的：

  Key key = KeyFactory.createKey（Chapter.class.getSimpleName（），chapterId）; 
章节chp = mgr.find（Chapter.class，key）; // chp始终为空（在调试模式下也是）
  

更新：

我在Book上尝试了同样的事情，它工作正常。所以问题在于章节。也许这是因为我通过Book保存了Chapter（但是我在上面提到的数据存储中都看到了）。

所以问题是：有没有一种方法可以独立检索章节通过它的关键字），如果是的话，一个代码片段请。

更新源代码：

  @Entity 
 public class Book实现java.io.Serializable {
 private static final long serialVersionUID = 1L; 
 
 @Id 
 @GeneratedValue（strategy = GenerationType.IDENTITY）
私钥键; 
 
 @OneToMany（cascade = CascadeType.ALL，fetch = FetchType.EAGER）
 private List< Chapter>章节= new ArrayList< Chapter>（）; 
 
公共列表< Chapter> getChapters（）{
返回章节; 
} 
 
 public void setChapters（List&Chapter&Chapters）{
 this.Chapters = Chapters; 
 
 
 public Book（long num，List< Chapter"章节）{
 super（）; 
 Key key = KeyFactory.createKey（Book.class.getSimpleName（），num）; 
 this.key = key; 
 this.Chapters =章节; 
} 
 
 public Book（long num）{
 super（）; 
 Key key = KeyFactory.createKey（Book.class.getSimpleName（），num）; 
 this.key = key; 
 
 $ b $ public Book（）{
} 
 
 public key getKey（）{
 return key; 
} 
 
 public void setKey（Key key）{
 this.key = key; 
} 
 
} 
 
 
 
 @实体
公共类实现java.io.Serializable {
 private static final long serialVersionUID = 1L; 
 
 @Id 
 @GeneratedValue（strategy = GenerationType.IDENTITY）
私钥键; 
 
私人字符串内容; 
 $ b $ public Chapter（String ChapterId，String content）{
 super（）; 
 Key key = KeyFactory.createKey（Chapter.class.getSimpleName（），ChapterId）; 
 this.key = key; 
 this.content = content; 
 
} 
 
 
公钥getKey（）{
 return key; 
} 
 
 public void setKey（Key key）{
 this.key = key; 
} 
 
 public String getContent（）{
 return content; 
} 
 
 public void set content（String content）{
 this.content = content; 
} 
 
 
} 
  

添加代码：

  Book bk = new Book（num）; 
章节chp =新章节（章节标识，内容）; 
 bk.getChapters（）。add（chp）; 
 bookDao.put（bk）; 
 
 mgr.persist（bk）; 
  

解决方案

我没有留下任何投票，但是您应该提供更多的周边代码。在您提供的代码中，大多数内容看起来都很好，但如果您在事务中创建书籍/章节（未显示），则该章节可能会将该书籍指定为父级，而您在手动指定时没有指定父级创建章节键。

I have two models, Book and Chapter, in a one-to-many relationship. I manually create the keys for both Book and Chapter. To persist, I create a book object then add an instance of chapter to it and then persist book. This works fine, as I see them in the datastore. Now when I try to fetch a chapter from the datastore by key, I get a null object.

Here is how the keys look in the datastore:

Under Book: name/id = 123    chapters = [Book(123)/Chapter("abc")]
Under Chapter: name/id = abc

I created my keys, both for creating and fetching objects, using

Key key = KeyFactory.createKey(Chapter.class.getSimpleName(), chapterId);

My fetching code is this:

Key key = KeyFactory.createKey(Chapter.class.getSimpleName(), chapterId);
Chapter chp = mgr.find(Chapter.class, key);//chp is always null (yes in debug mode as well)

UPDATE:

I try the same thing on Book and it works fine. So the problem is with Chapter. Perhaps it's because I saved Chapter through Book (but I see both in the datastore as mentioned above).

So the question is: Is there a way to retrieve chapter independently (by its key), if yes a code snippet please.

UPDATE source code:

@Entity
public class Book implements java.io.Serializable{
    private static final long serialVersionUID = 1L;

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Key key;

    @OneToMany(cascade = CascadeType.ALL, fetch = FetchType.EAGER)
    private List<Chapter> Chapters = new ArrayList<Chapter>();

    public List<Chapter> getChapters() {
        return Chapters;
    }

    public void setChapters(List<Chapter> Chapters) {
        this.Chapters = Chapters;
    }

    public Book(long num, List<Chapter> Chapters) {
        super();
        Key key = KeyFactory.createKey(Book.class.getSimpleName(), num);
        this.key = key;
        this.Chapters = Chapters;
    }

    public Book(long num) {
        super();
        Key key = KeyFactory.createKey(Book.class.getSimpleName(), num);
        this.key = key;
    }

    public Book() {
    }

    public Key getKey() {
        return key;
    }

    public void setKey(Key key) {
        this.key = key;
    }

}



@Entity
public class Chapter implements java.io.Serializable{
    private static final long serialVersionUID = 1L;

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Key key;

    private String content;

    public Chapter(String ChapterId, String content) {
        super();
        Key key = KeyFactory.createKey(Chapter.class.getSimpleName(), ChapterId);
        this.key = key;
        this.content = content;

    }


    public Key getKey() {
        return key;
    }

    public void setKey(Key key) {
        this.key = key;
    }

    public String getContent() {
        return content;
    }

    public void set content(String content) {
        this.content = content;
    }


}

Code for adding:

Book bk = new Book(num);
        Chapter chp = new Chapter(ChapterId, content);
        bk.getChapters().add(chp);
        bookDao.put(bk);

mgr.persist(bk);

解决方案

I did not leave any vote, but you should provide more of the surrounding code. Things mostly look fine in the code you gave, but if you created the book/chapter in a transaction (which isn't shown), the chapter may have the book specified as a parent, and you didn't specify a parent when manually creating the chapter key.

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