如何使用 Jpa 查询在 1 个 DTO 中加入 4 个表

Question

所以我有 4 张桌子

雇主

 @Entity

 @EqualsAndHashCode(callSuper = false)
 @Table(name = "employers")

 @NoArgsConstructor
 @AllArgsConstructor
 @JsonIgnoreProperties({"hibernateLazyInitializer", "handler", "jobPostings"})
 @PrimaryKeyJoinColumn(name="employer_id", referencedColumnName = "id")
 public class Employer extends User {
  
  @Column(name = "company_name")
  private String companyName;

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

  @Column(name = "phone_number")
  private String phoneNumber;
  
  @OneToMany(mappedBy="employer")
  private List<JobPosting> jobPostings;
  }

一个城市

@Data
@Entity
@Table(name="cities")
@AllArgsConstructor
@NoArgsConstructor
@JsonIgnoreProperties({"hibernateLazyInitializer", "handler", "jobPostings"})
public class City {

    @Id
    @Column(name="id")
    private int id;
    
    @Column(name="city_name")
    private String cityName;
    
    @OneToMany(mappedBy="city")
    private List<JobPosting> jobPostings;
}

一个职位

@AllArgsConstructor
@Data
@Entity
@Table(name="job_positions")
@JsonIgnoreProperties({"hibernateLazyInitializer", "handler", "jobPostings"})
public class JobPosition {
    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    @Column(name="job_position_id")
    private int id;
    
    @Column(name="title")
    private String title;
    
    @OneToMany(mappedBy="jobPosition")
    private List<JobPosting> jobPostings;
    
    
}

还有一个 JobPosting（如招聘广告）

@Entity
@Table(name="job_postings")
@Data
@NoArgsConstructor
@AllArgsConstructor
//@JsonIgnoreProperties({"hibernateLazyInitializer", "handler", "city", "jobPosition","employer"})

public class JobPosting {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name="id")
    private int id;
    
    
    
    @Column(name="job_requirements")
    private String jobRequirements;
    
    @Column(name="salary_min")
    private int salaryMin;
    
    @Column(name="salary_max")
    private int salaryMax;
    
    @Column(name="application_deadline")
    private LocalDate applicationDeadline;
    
    @Column(name="number_of_openings")
    private int numberOfOpenings;
    
    @Column(name="stream_date")
    private LocalDate streamDate;
    
    
    @ManyToOne()
    @JoinColumn(name="city_id")
    private City city;
    
    
    @ManyToOne()
    @JoinColumn(name="job_position_id")
    private JobPosition jobPosition;
    
    @ManyToOne()
    @JoinColumn(name= "employer_id")
    private Employer employer;
        
}

我已经为它们全部实现了必要的映射，并且工作正常。但是，我想加入他们的 DTO，例如：

@Data
@AllArgsConstructor
@NoArgsConstructor
public class JobPostingWithJobPositionCityEmployerDto {

    private int id;
    
    private String jobRequirements;

    private int salaryMin;

    private int salaryMax;

    private LocalDate applicationDeadline;

    private int numberOfOpenings;

    private LocalDate streamDate;

    private String cityName;

    private String title;

    private String companyName;
}

为了以更干净的方式获取我想要的字段，我正在尝试使用 springframework.jpa 的 @Query 注释，但我无法完全管理它，因为我刚刚了解到这个，我正在使用的查询是：

@Query(value ="Select new kodlamaio.hrms.entities.dtos.JobPostingWithJobPositionCityEmployerDto"
            + "(j.id, j.jobRequirements, j.salaryMin, j.salaryMax, j.numberOfOpenings, j.streamDate, j.applicationDeadline, c.cityName, p.title, e.companyName)"
            + " From Employer e Inner Join e.jobPostings j, "
            + "From City c Inner Join c.jobPostings j, "
            + "From JobPosition p Inner Join p.jobPostings j", nativeQuery = true)
    List<JobPostingWithJobPositionCityEmployerDto> getJobPostings();

我什至不知道这是否是正确的方法，我不断收到语法错误，我查找了答案但无法完全理解他们在说什么，有很多不同的场景。因此，如果有人可以帮助我完成此查询并推荐一些资源来了解不同的命令，我将不胜感激，在此先感谢。

Answer 1

在查询中使用 new 关键字将查询结果映射到 DTO 仅适用于 JPQL，它不适用于 SQL（您正在使用的）。

在我看来，如果您尝试编写过于复杂的查询，因为所有内容都可以通过 JobPosting class achieved/reached，这将在使用时隐式执行连接JPQL.

因此，而不是您的原生 SQL 编写 JPQL 应该可以修复它。

类似

@Query(value ="Select new kodlamaio.hrms.entities.dtos.JobPostingWithJobPositionCityEmployerDto"
            + "(jp.id, jp.jobRequirements, jp.salaryMin, jp.salaryMax, jp.numberOfOpenings, jp.streamDate, jp.applicationDeadline, jp.city.cityName, jp.jobPosition.title, jp. employer.companyName)"
            + " From JobPosting jp)

哪个应该可以解决问题。您的 JPA 提供程序应该足够聪明，能够确定要加入和检索的内容。

Answer 2

通常这被命名为 Projection，我相信您已经创建了 JobPostingWithJobPositionCityEmployerDto，其中包含 10 个参数的构造函数，当然还有它们各自的数据类型。由于它是 jpa 的习惯，因此您不能使用 nativeQuery = true。这些修改应该没问题。

@vlad-mihalcea 来救援 https://vladmihalcea.com/the-best-way-to-map-a-projection-query-to-a-dto-with-jpa-and-hibernate/

如何使用 Jpa 查询在 1 个 DTO 中加入 4 个表

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