Spring Jpa 我的实体的存储库查询 id 列两次

Question

I am working on a little Spring Boot app and I am having some issues understanding the Queries generated by JPA Repository.

I have 2 entities:

@Entity
@Getter
@Setter
@AllArgsConstructor
@NoArgsConstructor
@ToString(callSuper = true)
@EqualsAndHashCode(callSuper = true)
public class Project extends BaseEntity {
    @NotNull(message = "The project name is mandatory")
    private String name;
    private String description;

    @OneToMany(cascade = CascadeType.ALL, orphanRemoval = true, mappedBy = "project")
    private java.util.List<List> lists = new ArrayList<>();

    public void addList(List list) {
        lists.add(list);
        list.setProject(this);
    }

    public void removeList(List list) {
        lists.remove(list);
        list.setProject(null);
    }
}

@Entity
@Getter
@Setter
@AllArgsConstructor
@NoArgsConstructor
@ToString(callSuper = true)
@EqualsAndHashCode(callSuper = true)
public class List extends BaseEntity {
    private String name;

    @ManyToOne(fetch = FetchType.LAZY)
    private Project project;
}

And a base class they extend:

package com.projectmanager.models;

import lombok.EqualsAndHashCode;
import lombok.Getter;

import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.MappedSuperclass;
import java.io.Serializable;

@MappedSuperclass
@Getter
@EqualsAndHashCode
public class BaseEntity implements Serializable {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
}

I have created a repository for project entity:

package com.projectmanager.repositories;

import com.projectmanager.models.Project;
import com.projectmanager.projections.ProjectHeader;
import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.data.jpa.repository.Query;
import org.springframework.stereotype.Repository;

import java.util.List;

@Repository
public interface ProjectRepository extends JpaRepository<Project, Long> {
    @Query(
            "SELECT new com.projectmanager.projections.ProjectHeader( " +
                "p.id, " +
                "p.name," +
                "p.description" +
            " ) " +
            "FROM Project p")
    List<ProjectHeader> findAllProjectHeaders();
}

I am calling a service which is supposed to find a project and delete it from the DB.

public void deleteProject(Long projectId) {
    Project project = projectRepository.findById(projectId).orElseThrow(
            () -> new ProjectNotFoundException(projectId)
    );

    projectRepository.delete(project);
}

I have added in my application.properties these options:

spring.jpa.properties.hibernate.format_sql=true
logging.level.org.hibernate.SQL=DEBUG
logging.level.org.hibernate.type.descriptor.sql.BasicBinder=TRACE

When I try to call the above mentioned service, the queries printed in the console are:

select
    project0_.id as id1_1_0_,
    project0_.description as descript2_1_0_,
    project0_.name as name3_1_0_ 
from
    project project0_ 
where
    project0_.id=?

This first query corresponds to findById() call so everything is fine. The next one is:

select
    lists0_.project_id as project_3_0_0_,
    lists0_.id as id1_0_0_,
    lists0_.id as id1_0_1_,
    lists0_.name as name2_0_1_,
    lists0_.project_id as project_3_0_1_ 
from
    list lists0_ 
where
    lists0_.project_id=?

This is followed by delete queries which are irrelevant for this question.

The part that I am not understanding is why id gets selected twice from the DB in the query that searches for lists associated with the project:

lists0_.id as id1_0_0_,
lists0_.id as id1_0_1_,

Is this Spring specific or am I doing something wrong?

Answer 1

If you take a look on the internal implementation of delete method, it first checks if the record to be deleted exists in the table and then deletes the record. Hence extra select query