在Spring Data JPA中连接两个表实体

Question

@Entity
@Table(name = "Release_date_type")
public class ReleaseDateType {

    @Id
    @GeneratedValue(strategy=GenerationType.TABLE)
    private Integer release_date_type_id;
    // ...
    @Column(nullable = true) 
    private Integer media_Id;
    // with getters and setters...
}

另一个实体是：

@Entity
@Table(name = "Cache_Media")
public class CacheMedia {

    @Id
    @GeneratedValue(strategy=GenerationType.TABLE)
    private Integer id;
    // ...
    private Date loadDate; //with the getter and setter ..
}

我想编写一个CrudRepository接口，例如

public interface ReleaseDateTypeRepository extends CrudRepository<ReleaseDateType, Long>{
    @Query("SELECT * FROM Release_date_type a LEFT JOIN cache_media b on a.id=b.id")
    public List<ReleaseDateType> FindAllWithDescriptionQuery();
}

Answer 1

关于员工拥有一部或多部手机的典型例子，请参阅维基百科的这一部分。

对于您的特定示例，如果要进行一对一关系，则应该更改ReleaseDateType模型中的下一段代码：

@Column(nullable = true) 
private Integer media_Id;

对于：

@OneToOne(fetch = FetchType.LAZY)
@JoinColumn(name="CACHE_MEDIA_ID", nullable=true)
private CacheMedia cacheMedia ;

@OneToOne(cascade=ALL, mappedBy="ReleaseDateType")
private ReleaseDateType releaseDateType;

@Query("Select * from A a  left join B b on a.id=b.id")
public List<ReleaseDateType> FindAllWithDescriptionQuery();

//In this case a query annotation is not need since spring constructs the query from the method name
public List<ReleaseDateType> findByCacheMedia_Id(Integer id); 

@Query("FROM ReleaseDateType AS rdt WHERE cm.rdt.cacheMedia.id = ?1")    //This is using a named query method
public List<ReleaseDateType> FindAllWithDescriptionQuery(Integer id);

@Column(nullable = true) 
private Integer media_Id;

对于：

@OneToMany(cascade=ALL, mappedBy="ReleaseDateType")
private List<CacheMedia> cacheMedias ;

在CacheMedia模型中，您需要添加：

@ManyToOne(fetch = FetchType.LAZY)
@JoinColumn(name="RELEASE_DATE_TYPE_ID", nullable=true)
private ReleaseDateType releaseDateType;

然后在您的存储库中，您应该替换：

@Query("Select * from A a  left join B b on a.id=b.id")
public List<ReleaseDateType> FindAllWithDescriptionQuery();

//In this case a query annotation is not need since spring constructs the query from the method name
public List<ReleaseDateType> findByCacheMedias_Id(Integer id); 

@Query("FROM ReleaseDateType AS rdt LEFT JOIN rdt.cacheMedias AS cm WHERE cm.id = ?1")    //This is using a named query method
public List<ReleaseDateType> FindAllWithDescriptionQuery(Integer id);