[英]Fetch join by JPQL in openjpa for many to many mapping
我有一个device和device_group表,通过device_group_mapping表进行映射,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实体如下
@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;
}
由于获取类型是惰性的,因此我必须获取deviceGroupCollection如下代码
@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,当设备列表包含设备数量时,这将非常慢。
我想也许我可以通过JPQL来找到设备实体,并通过提取加入device_group,但不知道该怎么做。 根据openjpa规范,它不支持on子句,也不支持嵌套的访存联接。
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>
任何帮助表示赞赏。
您可以像在其他任何关联上一样使用访存联接ManyToMany。 你不需要任何on
化酶,因为关联映射已经定义了两个实体如何相互链接:
select d from Device d
left join fetch d.deviceGroupCollection
where ...
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