java 一对多关系的 JPQL
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JPQL for One to Many Relationship
提问by Ihsan Haikal
I'm trying to create a JPQL for one to many relationship, namely user and device as 1 user can have one to many devices. I want to find the whole object of the device if it is owned by the user and the device name is correct.
我正在尝试为一对多关系创建 JPQL，即用户和设备，因为 1 个用户可以拥有一对多设备。如果设备归用户所有并且设备名称正确，我想找到设备的整个对象。
If it is a SQL query then I can just do the query only for the device as follow:
如果是 SQL 查询，那么我可以只对设备进行查询，如下所示：
select * from DEVICE where USER_ID = 2 and DEVICENAME = "mypc";
where USER_ID is the foreign key in DEVICE table. But how can I do the JPQL query for the user and device table? Here are some info for User and Device class
其中 USER_ID 是 DEVICE 表中的外键。但是如何对用户和设备表进行 JPQL 查询？以下是用户和设备类的一些信息
@Entity
public class User {
@Id
@GeneratedValue(strategy = GenerationType.TABLE)
public int id;
public String username;
public String firstname;
public String lastname;
public String hashPassword;
@OneToMany(mappedBy = "user")
public List<Device> devices = new ArrayList<Device>();
}
@Entity
public class Device {
@Id
@GeneratedValue(strategy = GenerationType.TABLE)
public int id;
public String deviceName;
@ManyToOne
public User user;
String deviceOS;
String deviceType;
String attributes;
boolean standard;
}
回答by ozgur
Example JPA Query:Documention
示例 JPA 查询：文档
Query query = entityManager.createQuery("SELECT di FROM Device di JOIN di.user u WHERE u.id=:userId and di.deviceName=:deviceName");
query.setParameter("userId",userId );
query.setParameter("deviceName",deviceName);
List<Device> result = query.getResultList();
回答by Philipp Merkle
You can use JPQL path expressionsin your query to navigate from one entity to another, or to refer to attributes of an entity.
您可以在查询中使用JPQL 路径表达式从一个实体导航到另一个实体，或引用实体的属性。
More specifically, path expressions are useful to navigate along @ManyToOneor @OneToOnerelationships, e.g.:
更具体地说，路径表达式对于导航@ManyToOne或@OneToOne关系很有用，例如：
SELECT d FROM Device d WHERE d.user.id = :userId AND d.deviceName = :deviceName
Here, d.user.idis a path expression.
这里，d.user.id是一个路径表达式。
For a general overview of JPQL, have a look at the great Wikibooks article.
有关 JPQL 的一般概述，请查看很棒的Wikibooks 文章。
