Datastax Cassandra - 使用对象映射器时的继承

问题描述

我尝试使用Cassandra来维持相当简单的POJO，但使用类层次结构（许多子类，一个超类）。我使用Java和Datastax驱动程序（和对象映射器）。

我的问题是对象映射器似乎不识别超类中注释的字段。 p>

我实现的结构是：

  @Table（keyspace =。 ..，name =...）
 public class Subclass extends Superclass {
 public double x; 
 public double y; 
 public double z; 
 
 .... 
} 
 
 public class Superclass {
 @PartitionKey（0）
 @Column（name =user_id ）
 public long userId; 
 
 @PartitionKey（1）
 @Column（name =device_id）
 public long deviceId; 
} 
  

然后尝试保存子类对象：

  public static void main（String [] args）{
 Subclass data = new Subclass（）; 
 data.deviceId = 123; $ b $ d data.userId = 1212; 
 data.x = 0; 
 data.y = 1; 
 data.z = 2; 
 
集群集群; 
会话会话; 
 
 ... 
 
映射器<子类> mapper = new MappingManager（session）.mapper（Subclass.class）; 
 
 mapper.save（data）; 
} 
  

这会抛出：

  com.datastax.driver.core.exceptions.InvalidQueryException：缺少必需的PRIMARY KEY部分user_id 
  

有没有办法让它运行在这样的结构？一个解决方案是重新声明子类中的userId / deviceID字段，但这将是相当丑陋（很多重复的代码）。任何想法如何归档一个良好的结构或这种情况下的最佳实践？

解决方案

暂时不幸的是，你不能这个。您需要创建两个具有重复结构的单独类。

有一个票证添加支持，如果您想在驱动程序中看到此功能，请投票。 p>

I try using Cassandra for persisting quite simple POJOs, but using a class hierarchy (many subclasses, one superclass). I am using Java and the Datastax driver (and the object mapper).

My problem is that the Object Mapper doesn't seem to recognize fields annotated in superclasses.

The structure I implemented is:

@Table(keyspace = "...", name = "...")
public class Subclass extends Superclass {
    public double x;
    public double y;
    public double z;

   ....
}

public class Superclass {
    @PartitionKey(0)
    @Column(name = "user_id")
    public long userId;

    @PartitionKey(1)
    @Column(name = "device_id")
    public long deviceId;
}

I then try to save Subclass objects:

public static void main(String[] args) {
    Subclass data = new Subclass();
    data.deviceId = 123;
    data.userId = 1212;
    data.x = 0;
    data.y = 1;
    data.z = 2;

    Cluster cluster;
    Session session;

    ...

    Mapper<Subclass> mapper = new MappingManager(session).mapper(Subclass.class);

    mapper.save(data);
}

This throws:

 com.datastax.driver.core.exceptions.InvalidQueryException: Missing mandatory PRIMARY KEY part user_id

Is there any way to get it running in a structure like this? One solution was to redeclare the userId / deviceID field in the subclass, but this would be quite ugly (much repetitive code). Any ideas how to archive a good structure or best practices for a case like this?

解决方案

For the time being unfortunately you cannot do this. You would need to create two separate classes with a repetitive structure. You could create an interface with the shared fields if you want to address objects of both implementations in a common way.

There is a ticket JAVA-541 for adding support for this, please vote if you would like to see this capability in the driver.

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