Question

@Entity
@Table(name = "person")
public class Consignment implements Serializable {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

    @Column(name = "person_id")
    private String personId;

    @Column(name = "person_name")
    private String personName;

    @OneToMany(fetch = FetchType.EAGER, mappedBy = "person")
    @Column(name = "cars_owned")
    private Set<Cars> casrsowned = new HashSet<>();
}

@Entity
@Table(name = "cars")
public class Cars implements Serializable {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

   @Column(name = "cars_id")
    private String carsId; 


  @ManyToOne
    @JoinColumn(name = "person")
    private Person person;

    @OneToOne
    private CarsDetail  carsDetail;
}

@Entity
@Table(name = "carsDetail")
public class CarsDetail implements Serializable {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

    @Column(name = "brand")
    private String brand;

    @Column(name = "color")
    private String color;

    @Column(name = "model")
    private String model;
}

class CarModelDTO {

    String personName;
    List<String> models;
}

在上述关系中，要返回CarModelDTO JPA查询在哪里，

@Query("Select CarModelDTO(p.personName, p.casrsowned.carsDetail.model) from Person as p where p`enter code here`.id = :id"))
public CarModelDTO getCarmodelOwnedByAperson(@Param("id") Long id);

我尝试了多种方法，但是它给了 org.hibernate.QueryException: illegal attempt to dereference collection

Answer 1

正如我已经描述过的Retrieve List from repository interface to DTO list，您应该执行以下步骤：

首先使用要从查询输出中返回的字段创建构造函数
在查询中，您应该创建dto的新实例，并将字段从db传递到新的instalnce：

因此您需要进行以下更改：

1。在构造函数中：

您不应将列表用作List<String> models;，因为您应该将dto视为DB的结果行。所以您需要一个简单的String model;

    public CarModelDTO (String name,String model){    
        this.name=name;
        this.model=model;   
    }

2。在@Query中：

您应该适当地使用多重内部联接

您还应该在查询中将包名称附加到CarModelDTO（在这里我使用com.example，您应该更改它）

    @Query("Select com.example.CarModelDTO(p.personName, d.model ) from Person as p inner join p.carsowned c inner join c.carDetail d where p`enter code here`.id = :id")) 
    public CarModelDTO getCarmodelOwnedByAperson(@Param("id") Long id)

用集合加载DTO

1 个答案:

1。在构造函数中：

2。在@Query中：