Spring Data REST HATEOS : not lazy loading

Question

Problem

I have two entities defined: School and District. A district can have many schools and a school can belong to one district.
When executing a GET request against this endpoint http://localhost:8080/districts I would like to get a list of all the districts WITHOUT fetching each district's set of associated schools. But it seems like no matter what I do, hibernate is making DB calls to fetch data for each school individually.

Entities

School

@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;

} 

District

@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;

}

Log Output

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 = ? 

....

The SELECT FROM school as seen above, repeats hundreds of times despite having the schoolList configured as a lazy load.

Versions:

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

Finally figured this out ... I didn't include all of the code when I originally posted this question for simplicity sake. Unfortunately, the erroneous code was not something I originally posted.

What Was Happening

I had created a Projection for my School object and mapped the projection to the SchoolRepository as shown in the below code. I thought the projection was only applied when explicitly specified in a REST request (ie : /schools?projection=schoolExcerpt) But apparently, the projection was getting applied all the time. And for some reason, the District object was applying the SchoolProjection to each associated school -> resulting in SQL queries to fetch data form each school individually even with the @JsonIgore annotation present.

How I Solved the Issue

By simply removing the projection I was able to retrieve a list of all Districts without making thousands of calls to resolve each associated school object.

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

}

PS

I don't actually even need the @JsonIgnore annotation... HATEOAS is smart enough to not include related objects - instead it includes links to related objects.

Answer 2

Looks like your code even not workable, without @JsonIgnore for the field district during attempt to get all available schools you should recieve something like that:

com.fasterxml.jackson.databind.exc.InvalidDefinitionException: No serializer found for class org.hibernate.proxy.pojo.javassist.JavassistLazyInitializer and no properties discovered to create BeanSerializer (to avoid exception, disable SerializationFeature.FAIL_ON_EMPTY_BEANS) (through reference chain: java.util.ArrayList[0]->com.mberazouski.stackoverflow.springboothibernate.model.School["district"]->com.mberazouski.stackoverflow.springboothibernate.model.District_$$_jvstad5_0["handler"]) at com.fasterxml.jackson.databind.exc.InvalidDefinitionException.from(InvalidDefinitionException.java:77) ~[jackson-databind-2.9.6.jar:2.9.6]

But if you will add this annotation - everything should start working as expected. So the only change which should be added should be:

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

The result of trace will be:

2018-06-17 17:27:56.431 DEBUG 25024 --- [nio-8080-exec-1] org.hibernate.SQL : select school0_.id as id1_3_, school0_.closed as closed2_3_, school0_.district_id as district8_3_, school0_.enrollment as enrollme3_3_, school0_.grade_high as grade_hi4_3_, school0_.grade_low as grade_lo5_3_, school0_.name as name6_3_, school0_.number as number7_3_ from school school0_

In opposite to that if you will remove lazy:

@ManyToOne
@JoinColumn(name = "district_id")
private District district;

We will get full result:

Controller which was used for the test:

import com.mberazouski.stackoverflow.springboothibernate.model.School;
import com.mberazouski.stackoverflow.springboothibernate.repository.SchoolRepository;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

import java.util.List;

@RestController
public class SchoolController {
    @Autowired
    SchoolRepository schoolRepository;

    @GetMapping("/schools")
    public List<School> getAllSchools() {
        return schoolRepository.findAll();
    }
}

Hope that suggested change will solve your issue.

Good luck.