[英]Fetch join by JPQL in openjpa for many to many mapping
我有一個device和device_group表,通過device_group_mapping表進行映射,如下所示
CREATE TABLE device_group_mapping
(
device_id character varying(64) NOT NULL,
device_group_id bigint NOT NULL,
CONSTRAINT "FK_device_group_mapping_device" FOREIGN KEY (device_id)
REFERENCES device (id) MATCH SIMPLE
ON UPDATE NO ACTION ON DELETE NO ACTION,
CONSTRAINT "FK_device_group_mapping_device_group" FOREIGN KEY (device_group_id)
REFERENCES device_group (id) MATCH SIMPLE
ON UPDATE NO ACTION ON DELETE NO ACTION
)
WITH (
OIDS=FALSE
);
openjpa的device和deviceGroup實體如下
@Entity
@Table(name = "device")
public class Device implements Serializable
{
@ManyToMany(fetch = FetchType.LAZY)
@JoinTable(name = "device_group_mapping", joinColumns =
{@JoinColumn(name = "device_id", referencedColumnName = "id", nullable = false)}, inverseJoinColumns =
{@JoinColumn(name = "device_group_id", referencedColumnName = "id", nullable = false)})
private List<DeviceGroup> deviceGroupCollection;
}
@Entity
@Table(name = "device_group")
public class DeviceGroup implements Serializable
{
@ManyToMany(mappedBy = "deviceGroupCollection", fetch = FetchType.EAGER)
@OrderBy()
private List<Device> deviceCollection;
}
由於獲取類型是惰性的,因此我必須獲取deviceGroupCollection如下代碼
@Override
@Transactional
public List<Device> findAllDevicesWithGroupMapping() throws Exception
{
List<Device> list = new ArrayList<Device>();
list = this.deviceDao.findAll();
for (Device device : list)
{
device.setDeviceGroupCollection(device.getDeviceGroupCollection());
}
return list;
}
但是,當設備列表包含設備數量時,這將非常慢。
我想也許我可以通過JPQL來找到設備實體,並通過提取加入device_group,但不知道該怎么做。 根據openjpa規范,它不支持on子句,也不支持嵌套的訪存聯接。
我目前使用的openjpa如下
<dependency>
<groupId>org.apache.openjpa</groupId>
<artifactId>openjpa-all</artifactId>
<version>2.2.2</version>
</dependency>
任何幫助表示贊賞。
您可以像在其他任何關聯上一樣使用訪存聯接ManyToMany。 你不需要任何on
化酶,因為關聯映射已經定義了兩個實體如何相互鏈接:
select d from Device d
left join fetch d.deviceGroupCollection
where ...
聲明:本站的技術帖子網頁,遵循CC BY-SA 4.0協議,如果您需要轉載,請注明本站網址或者原文地址。任何問題請咨詢:yoyou2525@163.com.