如何通过id（Long值）检索Google Appengine对象？ [英] How to retrieve Google Appengine Objects by id (Long value)?

问题描述

我已经通过以下方式声明了一个实体：

  public class MyEntity {
 @PrimaryKey 
 @Persistent（valueStrategy = IdGeneratorStrategy.IDENTITY）
私人长ID; 
 @Persistent 
私人字符串文本; 
 // getters and setters 
} 
  

现在我想要检索对象使用ID。我尝试从Google Appengine数据查看器中通过SELECT * FROM MyEntity Where id = 382005或通过servlet中的查询进行管理。我没有得到任何结果。但我知道肯定与id的对象存在（我做了一个jsp查询数据库中的所有对象，并显示在数据库中）。

那么我的查询出了什么问题？我是否查询错误的字段？ Google Appengine Data Viewer将字段ID /名称命名为id = 382005。我必须用这个名字来查询吗？我已经尝试过，但没有成功：（$ / b>

解决方案

您可以在下面使用，因为您使用主键进行查询：

  MyEntity yourEntity = entityManager.find（MyEntity.class，yourId）; 
  

请注意，这应该可以工作，但是如果您基于主键进行搜索，则使用find（）会更容易：

  Query query = entityManager.createQuery（
SELECT m FROM MyEntity m WHERE id =：id）; 
 query.setParameter （id，yourId）; 
 
 MyEntity yourEntity =（MyEntity）query.getSingleResult（）; 
  

i have declared an entity the following way:

public class MyEntity {
 @PrimaryKey
 @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
 private Long id;
 @Persistent
 private String text;
        //getters and setters
}

Now I want to retrieve the objects using the id. I tried to manage it from the Google Appengine Data Viewer with "SELECT * FROM MyEntity Where id = 382005" or via a query in a servlet. I get no results returned. But i know for sure that the object with the id exists (i made a jsp which queries all objects in the db and displays them in the db).

So what is wrong in my query? Am I querying the wrong field? The Google Appengine Data Viewer names the field "ID/name" and it has the value "id=382005". Do I have to query with this names? I've tried but it didn't work out :(

解决方案

You can use below since you are querying using the primary key:

MyEntity yourEntity = entityManager.find(MyEntity.class, yourId);

Note, this should work as well, but it's easier to use find() if you are searching based on the primary key:

Query query = entityManager.createQuery(
    "SELECT m FROM MyEntity m WHERE id = :id");
query.setParameter("id", yourId);

MyEntity yourEntity = (MyEntity) query.getSingleResult();

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