從Spring Data Jpa查詢返回自定義對象

Question

我在jpa存儲庫類中有一個自定義查詢：

package it.univaq.we.internshipTutor.repository;
import ...

public interface ProfessorRepository extends JpaRepository<Professor, Long> {

    List<Professor> findAll();

    ...

    @Query(value =  "SELECT professor.id, professor.department_id, " +
                    "professor.first_name, professor.last_name, " +
                    "professor.email, COUNT(professor_id) as count " +
                    "FROM professor LEFT JOIN student_internship ON professor.id = professor_id " +
                    "GROUP BY professor_id ORDER BY count DESC LIMIT ?1", nativeQuery = true)
    List<ProfessorInternshipCount> mostRequestedProfessors(int limit);
}

該查詢返回10個最需要的實習輔導員/教授; 結果由教授的信息和整數值（計數）組成。

教授模特課：

package it.univaq.we.internshipTutor.model;

import ...

@Entity
@Table(name = "professor")
public class Professor {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id", nullable = false)
    private Long id;

    @Transient
    private UUID uuid;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "department_id", nullable = false)
    @NotNull(message = "this field is mandatory")
    private Department department;

    @OneToMany(fetch = FetchType.LAZY, mappedBy = "professor")
    private List<StudentInternship> studentInternships;

    @Column(name = "first_name", nullable = false, length = 255)
    @NotEmpty
    private String firstName;

    @Column(name = "last_name", nullable = false, length = 255)
    @NotEmpty
    private String lastName;

    @Column(name = "email", nullable = false, length = 255)
    @Email
    @NotEmpty
    private String email;

    ...getters and setters...
}

ProfessorInternshipCount模型（創建以封裝查詢結果）：

package it.univaq.we.internshipTutor.model;

public class ProfessorInternshipCount {
    private Professor professor;
    private Integer count;

    public ProfessorInternshipCount(Professor professor, int count) {
        this.professor = professor;
        this.count = count;
    }

    ...getters and setters...
}

現在，我很難將查詢返回的內容與我創建的模型綁定。 更准確地說，我得到以下例外：

org.springframework.core.convert.ConverterNotFoundException: 
    No converter found capable of converting from type 
    [org.springframework.data.jpa.repository.query.AbstractJpaQuery$TupleConverter$TupleBackedMap] 
    to type 
    [it.univaq.we.internshipTutor.model.ProfessorInternshipCount]
...

難道我做錯了什么？ 有沒有更好的方法來做我想做的事情？

Answer 1

您可以使用投影輕松實現此目的。 在這里你有以下列：

private String firstName;
private String lastName;
private Long id;

使用查詢中的getter創建一個接口。 您的預測將是這樣的：

public interface ITestProjection {
    Long getId();
    Integer getCount();
    String getFirstName();
    String getLastName();
}

您的查詢將是這樣的：

@Query(value = "SELECT professor.id, professor.department_id, " +
                    "professor.first_name, professor.last_name, " +
                    "professor.email, COUNT(professor_id) as count " +
                    "FROM professor LEFT JOIN student_internship ON professor.id = professor_id " +
                    "GROUP BY professor_id ORDER BY count DESC LIMIT =?1", nativeQuery = true)
    ArrayList<ITestProjection> findDataWithCount(Integer limit);

希望這能解決你的問題。

有關詳細信息，請訪問此主題 。

謝謝 ：）

Answer 2

您可以通過以下方式執行此操作。

1. 使用ModelMapper將實體轉換為目標bean類型
2. 使用Spring Projection來實現這一目標

閱讀文章以獲取更多詳細信息。

http://javasampleapproach.com/spring-framework/spring-data/query-alter-domain-model-spring-jpa-projection-springboot-mysql-database

Answer 3

例如，讓我們擁有：

• 用戶 - 具有許多字段的實體對象。

• UserBean - 只是我們的數據將被轉換的對象。

不好的做法：

 @Repository
    public class ReportingRepository {

        @PersistenceContext
        private EntityManager em;

        public List<UserBean> findQuery() {
            Query query = em.createNativeQuery("select  ...  from Table  INNER JOIN ...");
            List<UserBean> items = (List<UserBean>) query.getResultList();
            return items;
        }

    }

但它會返回數組中的值，所以如果我們編寫以下代碼會更漂亮，這是更好的做法：

query.unwrap(SQLQuery.class)
           .addScalar("instance name", StringType.INSTANCE)
           .addScalar("second instance name", IntegerType.INSTANCE)
           .setResultTransformer(Transformers.aliasToBean(UserBean.class));

   List<UserBean> items = query.getResultList();

所以最后一切都轉換為UserBean類。 請注意 ，在addScalar方法中，您應該傳遞實例變量名稱（在您的問題中，您有計數變量）