java - Spring 数据 REST HATEOAS : not lazy loading

Question

问题

我定义了两个实体:School 和 District。一个区可以有很多学校，一个学校可以属于一个区。
当针对此端点 http://localhost:8080/districts 执行 GET 请求时，我想获取所有学区的列表，而不获取每个学区的相关学校集.但似乎无论我做什么，hibernate 都会调用数据库来分别为每所学校获取数据。

实体

学校

@Getter
@Setter
@NoArgsConstructor
@Entity
public class School {

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

    @NotNull
    @Column(unique=true)
    private Long number;

    @NotNull
    @Column
    private String name;

    @NotNull
    private boolean closed;

    @Embedded
    private ContactInfo contactInfo;

    private String gradeLow;
    private String gradeHigh;
    private int enrollment;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "district_id")
    private District district;

} 

区

@Getter
@Setter
@NoArgsConstructor
@Entity
public class District {

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

    @Column(unique = true)
    private Integer number;

    private String  name;
    private String  type;
    private int     enrollment;
    private Date    updated;

    @Embedded
    private ContactInfo contactInfo;

    @Getter(AccessLevel.NONE)
    @JsonIgnore
    @OneToMany(fetch = FetchType.LAZY, mappedBy = "district")
    private Set<School> schoolList;

}

日志输出

SELECT district0_.id          AS id1_5_, 
       district0_.city        AS city2_5_, 
       district0_.email       AS email3_5_, 
       district0_.fax         AS fax4_5_, 
       district0_.first_name  AS first_na5_5_, 
       district0_.last_name   AS last_nam6_5_, 
       district0_.name_prefix AS name_pre7_5_, 
       district0_.phone       AS phone8_5_, 
       district0_.state       AS state9_5_, 
       district0_.street      AS street10_5_, 
       district0_.title       AS title11_5_, 
       district0_.website     AS website12_5_, 
       district0_.zip         AS zip13_5_, 
       district0_.enrollment  AS enrollm14_5_, 
       district0_.NAME        AS name15_5_, 
       district0_.number      AS number16_5_, 
       district0_.type        AS type17_5_, 
       district0_.updated     AS updated18_5_ 
FROM   district district0_ 

SELECT schoollist0_.district_id AS distric20_7_0_, 
       schoollist0_.id          AS id1_7_0_, 
       schoollist0_.id          AS id1_7_1_, 
       schoollist0_.closed      AS closed2_7_1_, 
       schoollist0_.city        AS city3_7_1_, 
       schoollist0_.email       AS email4_7_1_, 
       schoollist0_.fax         AS fax5_7_1_, 
       schoollist0_.first_name  AS first_na6_7_1_, 
       schoollist0_.last_name   AS last_nam7_7_1_, 
       schoollist0_.name_prefix AS name_pre8_7_1_, 
       schoollist0_.phone       AS phone9_7_1_, 
       schoollist0_.state       AS state10_7_1_, 
       schoollist0_.street      AS street11_7_1_, 
       schoollist0_.title       AS title12_7_1_, 
       schoollist0_.website     AS website13_7_1_, 
       schoollist0_.zip         AS zip14_7_1_, 
       schoollist0_.district_id AS distric20_7_1_, 
       schoollist0_.enrollment  AS enrollm15_7_1_, 
       schoollist0_.grade_high  AS grade_h16_7_1_, 
       schoollist0_.grade_low   AS grade_l17_7_1_, 
       schoollist0_.NAME        AS name18_7_1_, 
       schoollist0_.number      AS number19_7_1_ 
FROM   school schoollist0_ 
WHERE  schoollist0_.district_id = ? 

SELECT schoollist0_.district_id AS distric20_7_0_, 
       schoollist0_.id          AS id1_7_0_, 
       schoollist0_.id          AS id1_7_1_, 
       schoollist0_.closed      AS closed2_7_1_, 
       schoollist0_.city        AS city3_7_1_, 
       schoollist0_.email       AS email4_7_1_, 
       schoollist0_.fax         AS fax5_7_1_, 
       schoollist0_.first_name  AS first_na6_7_1_, 
       schoollist0_.last_name   AS last_nam7_7_1_, 
       schoollist0_.name_prefix AS name_pre8_7_1_, 
       schoollist0_.phone       AS phone9_7_1_, 
       schoollist0_.state       AS state10_7_1_, 
       schoollist0_.street      AS street11_7_1_, 
       schoollist0_.title       AS title12_7_1_, 
       schoollist0_.website     AS website13_7_1_, 
       schoollist0_.zip         AS zip14_7_1_, 
       schoollist0_.district_id AS distric20_7_1_, 
       schoollist0_.enrollment  AS enrollm15_7_1_, 
       schoollist0_.grade_high  AS grade_h16_7_1_, 
       schoollist0_.grade_low   AS grade_l17_7_1_, 
       schoollist0_.NAME        AS name18_7_1_, 
       schoollist0_.number      AS number19_7_1_ 
FROM   school schoollist0_ 
WHERE  schoollist0_.district_id = ? 

SELECT schoollist0_.district_id AS distric20_7_0_, 
       schoollist0_.id          AS id1_7_0_, 
       schoollist0_.id          AS id1_7_1_, 
       schoollist0_.closed      AS closed2_7_1_, 
       schoollist0_.city        AS city3_7_1_, 
       schoollist0_.email       AS email4_7_1_, 
       schoollist0_.fax         AS fax5_7_1_, 
       schoollist0_.first_name  AS first_na6_7_1_, 
       schoollist0_.last_name   AS last_nam7_7_1_, 
       schoollist0_.name_prefix AS name_pre8_7_1_, 
       schoollist0_.phone       AS phone9_7_1_, 
       schoollist0_.state       AS state10_7_1_, 
       schoollist0_.street      AS street11_7_1_, 
       schoollist0_.title       AS title12_7_1_, 
       schoollist0_.website     AS website13_7_1_, 
       schoollist0_.zip         AS zip14_7_1_, 
       schoollist0_.district_id AS distric20_7_1_, 
       schoollist0_.enrollment  AS enrollm15_7_1_, 
       schoollist0_.grade_high  AS grade_h16_7_1_, 
       schoollist0_.grade_low   AS grade_l17_7_1_, 
       schoollist0_.NAME        AS name18_7_1_, 
       schoollist0_.number      AS number19_7_1_ 
FROM   school schoollist0_ 
WHERE  schoollist0_.district_id = ? 

SELECT schoollist0_.district_id AS distric20_7_0_, 
       schoollist0_.id          AS id1_7_0_, 
       schoollist0_.id          AS id1_7_1_, 
       schoollist0_.closed      AS closed2_7_1_, 
       schoollist0_.city        AS city3_7_1_, 
       schoollist0_.email       AS email4_7_1_, 
       schoollist0_.fax         AS fax5_7_1_, 
       schoollist0_.first_name  AS first_na6_7_1_, 
       schoollist0_.last_name   AS last_nam7_7_1_, 
       schoollist0_.name_prefix AS name_pre8_7_1_, 
       schoollist0_.phone       AS phone9_7_1_, 
       schoollist0_.state       AS state10_7_1_, 
       schoollist0_.street      AS street11_7_1_, 
       schoollist0_.title       AS title12_7_1_, 
       schoollist0_.website     AS website13_7_1_, 
       schoollist0_.zip         AS zip14_7_1_, 
       schoollist0_.district_id AS distric20_7_1_, 
       schoollist0_.enrollment  AS enrollm15_7_1_, 
       schoollist0_.grade_high  AS grade_h16_7_1_, 
       schoollist0_.grade_low   AS grade_l17_7_1_, 
       schoollist0_.NAME        AS name18_7_1_, 
       schoollist0_.number      AS number19_7_1_ 
FROM   school schoollist0_ 
WHERE  schoollist0_.district_id = ? 

....

尽管将 schoolList 配置为延迟加载，但如上所示，SELECT FROM school 重复了数百次。

版本:

springBootVersion = '1.4.2.RELEASE'  
hibernate-core:5.0.11
'org.springframework.boot:spring-boot-starter-data-jpa'
'org.springframework.boot:spring-boot-starter-data-rest'
'org.springframework.boot:spring-boot-starter-web'
'org.springframework.boot:spring-boot-starter-actuator'
'org.springframework.boot:spring-boot-starter-hateoas'
'org.springframework.boot:spring-boot-starter-security'

Answer 1

终于想通了......为了简单起见，我最初发布这个问题时没有包含所有代码。不幸的是，错误代码不是我最初发布的。

发生了什么

我创建了一个 Projection对于我的 School 对象，并将投影映射到 SchoolRepository，如下面的代码所示。我认为只有在 REST 请求中明确指定时才会应用投影(即:/schools?projection=schoolExcerpt)但显然，投影一直在应用。出于某种原因，District 对象将 SchoolProjection 应用于每个关联的学校 -> 导致 SQL 查询从每个学校单独获取数据，即使使用 @ JsonIgore 注释存在。

我是如何解决这个问题的

通过简单地删除投影，我能够检索所有 Districts 的列表，而无需进行数千次调用来解析每个关联的学校对象。

@RepositoryRestResource(excerptProjection = SchoolProjection.class)  //removing this line solves my problems
public interface SchoolRepository extends CrudRepository<School, Long>{    

}

附言

我什至不需要 @JsonIgnore 注释...HATEOAS 足够聪明，不会包含相关对象 - 相反，它包含指向相关对象的链接。