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'

Urinalysis

@JoinColumn(name = "district_id") this should be the id of the district entity. I don't see a mapping of the district_id in the district entity. – John

Have you exposed and endpoint for schools? If so, SDR should not include the school data but instead there should be a link to get the data. This data is exposed via a SDR Rest repository rather than a Spring MVC controller? Additionally, you have no other Lombok annotations on your entity classes. Is that correct? – Alvaalvan

@Niver that's the beauty of hibernate. Its implicitly joins with the ID column, you dont need matching column names. – Investiture

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.

Urinalysis

This means, you ahve to add default projection on caller`s side. I hope this will help: jira.spring.io/browse/DATAREST-1089 – Ternate

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.

Jaclynjaco

Answer 3

In your case, you either have to create another VIEW for districts service call or you wanna try the @JSONIGNORE for schoolListproperty to have LazyLoad in effect (I am guessing your presentation layer is JSON).

What's happening is, when you are making a districts service call, its returning the District object with lazyload ofcourse, but as soon as your POJO to JSON transformation is handled by controller, it is calling School getter and then further method of School object to create full data representation causing hibernate to load school objects as well.

UPDATE

Added sample code for you in github to refer.

You can choose between three options:

• @JsonIgnore - will completely ignore the property for any serialization/deserialization. Not sure if that's what you really want it.
• @JsonManagedReference & @JsonManagedReference - will avoid json recursion/stackoverflow issue due to hibernate/table cross referencial structure.
• @JsonIgnoreProperties - another way to avoid json recursion issue for referencial entities.

NOTE: FetchType.LAZY is by default, so haven't explicitly stated it.

Code snippet for quick reference:

@Getter
@Setter
@Entity
public class District {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    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;

    @OneToMany(mappedBy = "district")
    //@JsonManagedReference //to avoid JSON recursion solution 1
    //@JsonIgnoreProperties("district") //to avoid JSON recursion solution 2
    @JsonIgnore
    private Set<School> schools;

}


@Getter
@Setter
@Entity
public class School {

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

    @Column(unique=true, nullable = false)
    private Long number;

    @Column(nullable = false)
    private String name;

    private boolean closed;

    @Embedded
    private ContactInfo contactInfo;

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

    @ManyToOne
    @JoinColumn(name = "district_id", nullable = false)
    //@JsonBackReference //to avoid JSON recursion solution 1
    //@JsonIgnoreProperties("schools") //to avoid JSON recursion solution 2
    @JsonIgnore
    private District district;

} 

Investiture

What you're saying about the the POJO to JSON transformation calling the school getter makes sense. But I'm oddly getting the same results even after adding @JsonIgnore and @Getter(AccessLevel.NONE) to remove the getter. – Urinalysis

Updated the answer for you with sample code in github, you can checkout & refer/run at your local. It is following the lazyload. – Investiture

The question has the Spring Data Rest tag. Your answer and example relate to returning JSON data via Spring MVC which is something completely different. – Alvaalvan