java - 实体属性可以被同一实体的其他属性映射吗？

Question

我有一个如下所示的实体结构:

@Entity
public class Event {
     @Id
     private Long id;
     @ManyToOne
     private Device device;
     @Column
     private Severity severity;

     ... getters/setters/other attrs ...
}

@Entity
public class Device {
     @Column
     private Impact impact;
     @ManyToOne
     private PriorityMatrix priorityMatrix;
     ... getters/setters/other attrs ...
}

@Entity
public class Priority {
     @EmbeddedId
     private PriorityId id;
     @Column
     private Long value;    
     ... getters/setters ...
}

@Embeddable
public class PriorityId {
     @Column
     private Severity severity;
     @Column
     private Impact impact;
     @ManyToOne
     private PriorityMatrix matrix;
     ... getters/setters ...
}

影响和严重性是具有固定值的枚举。

我可以向按设备影响和优先级矩阵以及事件严重性映射的事件实体添加“ transient ”属性“优先级”吗？如果是，怎么办？

在 SQL 中，这将是一些连接，例如

SELECT priority_matrix.priority_value, 
      -- event attributes
      -- device attributes
    FROM event
    INNER JOIN device ON { -- event x device join }
    INNER JOIN priority_matrix ON {
         device.priority_matrix_id = priority_matrix.id
         AND device.impact = priority_matrix.impact
         AND event.severity = priority_matrix.severity
    }

我想这样做是因为优先级矩阵可以由不同设备更新和共享，因此在获取事件时必须始终获取优先级值，但我想在加载事件时加载优先级值。

Answer 1

你可以有 transient 对象，但它不会对你的数据库生效，例如在生成的sql中。

//Event class:
@Transient
private Priority priority;


//setter
public void setPriority(Priority p){
    //if you need to change/update the persistent object when setting a Priority, 
//you could set the mapped fields in this setter
    this.device.setImpact(p.getId().getImpact());
    this.setServerity(p.getId().getSeverity());
}

//getter 
public Priority getPriority(){
//since annotated by Transient annotation,this will NOT return persistent object. 
// but you could get the transient Priority object and then load it in future
    Priority p = new Priority();
    //create a PriorityId object
    priorityId.set(..this.device..this.severity..)
    p.setPriorityId(priorityId);
    p.setValue(...);
    ...
    return p; //transient object!
}

上面的代码不是用IDE编写的，可能有错别字，但它表达了想法。