在 Spring Data Jpa 中使用 JPQL 连接查询

Question

我在 spring data jpa 中使用JPQL 创建了一个left join 查询，但在我的单元测试中失败了。项目中有两个实体。

Product实体：

@Entity
@Table(name = "t_goods")
public class Product implements Serializable {

    @Id
    @GeneratedValue
    @Column(name = "id", length = 6, nullable = false)
    private Integer id;
    @Column(name = "name", length = 20, nullable = false)
    private String name;
    @Column(name = "description")
    private String desc;
    @Column(name = "category", length = 20, nullable = false)
    private String category;
    @Column(name = "price", nullable = false)
    private double price;
    @Column(name = "is_onSale", nullable = false)
    private Integer onSale;

    @ManyToOne(cascade = CascadeType.ALL)
    @JoinColumn(name = "brand_id")
    private Brand brand;
    // getter and setter
}

Brand实体：

@Entity
@Table(name = "tdb_goods_brand")
public class Brand implements Serializable {

    @Id
    @GeneratedValue
    @Column(name = "id", length = 6, nullable = false)
    private  Integer id;
    @Column(name = "brand_name", unique = true, nullable = false)
    private String name;

    @OneToMany(mappedBy = "brand", fetch = FetchType.EAGER, cascade = CascadeType.ALL)
    private List<Product> products;
    // getter and setter
}

还有一个第三类Prod将查询结果映射到Object：

public class Prod implements Serializable {
    private Integer id;
    private String name;
    private double price;
    //private String brandName;
    // getter and setter
}

这个查询很好用：

public interface ProductRepository extends JpaRepository<Product, Integer> {
    @Query(value = "select new com.pechen.domain.Prod(p.id, p.name, p.price) from Product p ")
    Page<Prod> pageForProd(Pageable pageRequest);
}

但如果我为Prod 添加新属性brandName 并使用left join 重构查询，则测试失败：

@Query(value = "select new com.pechen.domain.Prod(p.id, p.name, p.price, b.name) from Product p left join com.pechen.domain.Brand b on p.brand_id = b.id")
Page<Prod> pageForProd(Pageable pageRequest);

问题似乎出在on p.brand_id = b.id，因为Product 中没有brand_id 属性，它只是一个列名。那么我怎样才能完成这项工作呢？

更新：

JPQL 查询中出现了一些语法错误，只需将其修复如下：

@Query(value = "select new com.pechen.domain.Prod(p.id, p.name, p.price, b.name) from Product p left join p.brand b")
Page<Prod> pageForProd(Pageable pageRequest);

此外，这种方式每次都创建另一个类将查询结果映射到对象（我的意思是Prod类）非常麻烦。那么有没有好的方法来使用它呢？任何帮助将不胜感激。

Answer 1

您应该使用p.brand.id = b.id 而不是p.brand_id = b.id