Fetch join by JPQL in openjpa for many to many mapping

Question

我有一个设备和 device_group table，通过 device_group_mapping table 映射如下

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，我必须按照下面的代码获取 deviceGroupCollection

@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，当设备列表包含大量设备时，这将非常慢。

我想也许我可以通过 JPQL 和 fetch join device_group 找到设备实体，但不知道该怎么做。根据openjpa规范，它不支持on子句和嵌套fetch join。

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>

感谢任何帮助。

Answer 1

您可以像在任何其他关联上一样使用 Fetch Join 和 ManyToMany。您不需要任何 on 类，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 ...

Fetch join by JPQL in openjpa for many to many mapping

Fetch join by JPQL in openjpa for many to many mapping

java

mapping

jpa

openjpa

fetch