hibernate - 多对多与连接表，sql 查询不连接

Question

我有以下两个 POJO：

@Entity
@Table
@Data
@EqualsAndHashCode(of={"identifier"})
@ToString(of={"identifier"})
public class Project {
    @Column
    @Id
    @GeneratedValue
    private Integer identifier;

    @OneToMany(cascade = CascadeType.ALL, mappedBy = "primaryKey.project")
    private Set<MemberToProject> members = new HashSet<MemberToProject>();
}

@Entity
@Data
@Table
@EqualsAndHashCode(of={"identifier"})
@ToString(of={"identifier"})
public class Member {
    @Column
    @Id
    @GeneratedValue
    private Integer identifier;

    @OneToMany(cascade = CascadeType.ALL, mappedBy = "primaryKey.member")
    private Set<MemberToProject> projects = new HashSet<MemberToProject>();
}

连接表的POJO如下：

@Entity
@Table
@Data
public class MemberToProject {
    @Embeddable
    @Data
    public static class PrimaryKey implements Serializable {
        @ManyToOne
        private Member member;

        @ManyToOne
        private Project project;
    }

    @Column
    @EmbeddedId
    private PrimaryKey primaryKey;
}

我想要一个单独的 POJO 用于连接表，这样我就可以添加属性，例如成员加入项目的日期、成员的权限等。为简洁起见，我省略了。

我用一些数据填充我的数据库并执行以下代码：

final Session s = ...;
final Member m = (Member) s.load(Member.class, 3);

System.out.println(m.getProjects());

Hibernate 生成以下三个查询：

Hibernate: 
    select
        member0_.identifier as identifi1_1_0_,
        member0_.name as name2_1_0_ 
    from
        Member member0_ 
    where
        member0_.identifier=?
Hibernate: 
    select
        projects0_.member_identifier as member1_1_1_,
        projects0_.member_identifier as member1_4_1_,
        projects0_.project_identifier as project2_4_1_,
        projects0_.member_identifier as member1_4_0_,
        projects0_.project_identifier as project2_4_0_ 
    from
        MemberToProject projects0_ 
    where
        projects0_.member_identifier=?
Hibernate: 
    select
        project0_.identifier as identifi1_6_0_,
        project0_.name as name2_6_0_ 
    from
        Project project0_ 
    where
        project0_.identifier=?

Hibernate 在此处生成低效查询。 membertoproject 中的 project 和 member 之间没有连接。我相信第二个和第三个查询可以/应该结合起来。我怎样才能做到这一点？

另外，第二个查询多次检索同一列，出于某种原因，我不明白。

Answer 1

我认为，“成员”和“项目”之间的关系是多对多。

试试看

@Entity
@Table
@Data
@EqualsAndHashCode(of={"identifier"})
@ToString(of={"identifier"})
public class Project {
@Column(name="PROJECT_ID")
@Id
@GeneratedValue
private Integer projectId;

@ManyToMany(cascade = CascadeType.ALL, mappedBy = "projects")
private Set<MemberToProject> members = new HashSet<MemberToProject>();
}

@Entity
@Data
@Table
@EqualsAndHashCode(of={"identifier"})
@ToString(of={"identifier"})
public class Member {
@Column(name="MEMBER_ID")
@Id
@GeneratedValue
private Integer memberId;

@ManyToMany(cascade = {CascadeType.ALL})
@JoinTable(name="MEMBER_PROJECT", 
            joinColumns={@JoinColumn(name="PROJECT_ID")}, 
            inverseJoinColumns={@JoinColumn(name="MEMBER_ID")})
private Set<MemberToProject> projects = new HashSet<MemberToProject>();
}