Java 如何处理JPA多对一关系？

我正在设计一个用于收集天气数据的应用程序。我有两个POJO对象“位置”和“记录”。位置包含有关纬度和经度以及当前天气状况的信息，记录包含特定位置随时间变化的所有天气信息，因此与位置存在多对一关系。我的课程定义如下：

Location.java

@Entity
@Table(name = "location")
@NamedQueries( {
 @NamedQuery(name = "findLocations", query = "SELECT e FROM Location e ORDER BY e.longitude, e.latitude"),
 @NamedQuery(name = "findLocationByLatLong", query = "SELECT e from Location e WHERE e.latitude = :latitude AND e.longitude = :longitude"),
 @NamedQuery(name = "findLocationById", query = "SELECT e from Location e WHERE e.id = :id"),
 @NamedQuery(name = "deleteLocationById", query= "DELETE Location e WHERE e.id = :id"),
 @NamedQuery(name = "updateLocation", query = "UPDATE Location e SET e.lastModifiedDate = :lastModifiedDate WHERE e.id = :id")})

public class Location implements Serializable {

 /**
  * 
  */
 private static final long serialVersionUID = 1L;

 @Id
 @GeneratedValue(strategy = GenerationType.AUTO)
 protected Long id;

 @Column(name="latitude", nullable=false)
 protected String latitude;

 @Column(name="longitude", nullable=false)
 protected String longitude;

 @Column(name="lastModifiedDate")
 @Temporal(TemporalType.TIMESTAMP)
 private Date lastModifiedDate;

 @Column(name="windDirection")
 private float windDirection;

 @Column(name="windSpeed")
 private float windSpeed;

 @Column(name="temperature")
 private float temperature;
}

和Record.java

@Entity
@Table(name = "weatherdata")
@NamedQueries( {
  @NamedQuery(name = "findWeatherRecordById", query = "SELECT e from Record e WHERE e.id = :id"),
  @NamedQuery(name = "findWeatherRecords", query = "SELECT e from Record e WHERE e.parent = :parent") })
public class Record implements Serializable{

 /**
  * 
  */
 private static final long serialVersionUID = 1L;

 @Id
 @GeneratedValue(strategy = GenerationType.AUTO)
 @Column(name="id")
 protected Long id;

 @Column(name="mTime")
 @Temporal(TemporalType.TIMESTAMP)
 private Date mtime;

 @Column(name="windDirection")
 private float windDirection;

 @Column(name="windSpeed")
 private float windSpeed;

 @Column(name="temperature")
 private float temperature;

 @ManyToOne(cascade = { CascadeType.ALL }, targetEntity=Location.class, fetch = FetchType.EAGER)
 @JoinColumn(name="locationId")
 protected Location parent;
}

我的输入数据的形式如下：

latitude,longitude,date,winddirection,windspeed,temperature
36.9822,-122.0153,20100907000000.00,158,2.68,20.57
38.1838,-120.54,20100907000000.00,248,0,26.68
38.3495,-121.9688,20100907000000.00,149,0.45,33.9
38.41935,-121.36029,20100907000000.00,322,0.9,33.9
37.91617,-122.286,20100907000000.00,224,0,24.46
38.587,-121.3162,20100907000000.00,315,0,34.46
36.8717,-121.6555,20100907000000.00,294,3.13,18.34

现在，每当我得到一条记录，我都想把它插入记录表中。由于我有一个指向Location的外键，我还将添加Location表的locationId。 另一件事，位置表不是预先填充的。所以，每当出现新记录时，我首先将其插入位置表，然后用外键填充记录表。我不想在位置表中重复位置项。位置表还将包含您可以看到的最新温度、风速和风向数据

我使用以下代码来实现这一点：

Location loc = handler.getLocation(line);   
//loc.setTemperature(0);

Location dbLoc = null;

try {
    Query q = eManager.createNamedQuery("findLocationByLatLong");
    q.setParameter("latitude", loc.getLatitude());
    q.setParameter("longitude", loc.getLongitude());
    dbLoc = (Location) q.getSingleResult();                     
} catch (Exception e) {
    System.out.println("Location not found! Creating new location");
    Logger.getLogger("WeatherRecorderBean.class").log(Level.WARNING, e.getMessage());
}


Record r = handler.getRecord(line);

if(dbLoc!=null) {
    r.setParent(dbLoc);

    dbLoc.setLastModifiedDate(r.getMtime());//I am doing this so as to know what time the weather change entry is about
    dbLoc.setWindDirection(r.getWindDirection());
    dbLoc.setWindSpeed(r.getWindSpeed());
    dbLoc.setTemperature(r.getTemperature());

    eManager.merge(r);                      
}
else {
    dbLoc = new Location();
    dbLoc.setLatitude(loc.getLatitude());
    dbLoc.setLongitude(loc.getLongitude());
    //eManager.persist(dbLoc);

    r.setParent(dbLoc);                     

    dbLoc.setLastModifiedDate(r.getMtime());
    dbLoc.setWindDirection(r.getWindDirection());
    dbLoc.setWindSpeed(r.getWindSpeed());
    dbLoc.setTemperature(r.getTemperature());

    eManager.merge(r);
    //eManager.merge(dbLoc);

}

但通过这样做，正在发生的是位置被复制。这意味着我在位置表中有多个相同经纬度但不同温度、风速数据的条目。我想做的是为一个纬度和经度设置一个条目，并用最新数据更新风速、温度和风向字段

---

请帮忙

您将

所有
操作从
记录
级联到
位置
，因此当您合并新的
记录
和它的新父记录
位置

时，无需再次合并临时的

位置

（否则您将得到重复的行）

我在下面的代码中添加了一些注释（我并没有修复所有问题，还有更多问题，但建议的更改至少应该删除重复项的创建

Location

条目）：

已解决：-）

我创建了一个用于保存

位置的新函数。
我也有一些同步问题。我在MDB
onMessage（）函数中有这个函数。因此，在一个
onMessage（）
完成之前，另一个开始创建重复条目

希望这对将来有帮助
 嗨，谢谢你的帮助。我更改了代码（也相应地更新了问题中的上述代码）。但它仍然会给我重复的条目（@Vashishtha我可能遗漏了什么，但至少上面的说法不正确。激活SQL日志记录以查看发生了什么。我不能为您这样做：）@Pascal I启用了SQL日志记录，我看到的只是一系列insert语句：（如果任何列的任何单个值不同，则会将新行相互关联。@Vashishtha哪个部分没有按预期工作？当位置已经存在时？当它不存在时？两者都有？@Pascal两者都有。我尝试保存位置以供检查。但这也会在位置表中创建重复项。
if(dbLoc==null) {
  dbLoc = new Location();
  dbLoc.setLatitude(loc.getLatitude());
  dbLoc.setLongitude(loc.getLongitude());
  r.setParent(dbLoc);

  // add changes on the dbLoc here

  eManager.merge(r);  // here you're merging r and the dbLoc 

  //loc.setLastModifiedDate(r.getMtime()); // why are you modifying the line from the file?
  //loc.setWindDirection(r.getWindDirection());
  //loc.setWindSpeed(r.getWindSpeed());
  //loc.setTemperature(r.getTemperature());
  //eManager.persist(loc);    
  //System.out.println("Location id : "+loc.getId());

  //eManager.merge(dbLoc); // here you're merging a transient dbLoc again

}

public Location saveLocation(Location loc) {
    eManager.clear();
    eManager.setFlushMode(FlushModeType.COMMIT);
    //eManager.setFlushMode(FlushModeType.COMMIT);
    Query q = eManager.createNamedQuery("findLocationByLatLong");
    q.setParameter("latitude", loc.getLatitude());
    q.setParameter("longitude", loc.getLongitude());
    try {
        Location dummy = (Location) q.getSingleResult();
        eManager.clear();
        // eManager.flush();
        return dummy;
    } catch (NoResultException ex) {
        Logger.getLogger("WeatherRecorderBean.class").log(Level.WARNING,
                ex.getMessage());
        eManager.clear();
        eManager.merge(loc);
        eManager.flush();
        return loc;
    } catch (Exception ex) {
        Logger.getLogger("WeatherRecorderBean.class").log(Level.WARNING, "Should never get here! "+ex.getMessage());
        return null;
    }
}