OneToMany
![relation]()
위 처럼 일대다 관계를 가지는 Entity를 작성하는 방법과 차이점에 대한 정리
기본 동작(by JoinTable)
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| @Entity
class Student(
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
var id: Long,
var name: String
)
@Entity
class Class(
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
var id: Long,
@OneToMany
var students: MutableSet<Student> = mutableSetOf()
)
|
결과
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| create table class (
id bigint generated by default as identity,
primary key (id)
)
create table class_students (
class_id bigint not null,
students_id bigint not null,
primary key (class_id, students_id)
)
create table student (
id bigint generated by default as identity,
name varchar(255),
primary key (id)
)
|
![join-table]()
OneToMany관계는 기본적으로 JoinTable 기반으로 동작하며 생성되는 Table은 위와 같다.
MappedBy
자식(여기서는 student) entity에서 부모 entity의 정보를 포함하여 ManyToOne 관계를 구현
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| @Entity
class Student(
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
var id: Long,
var name: String,
var classId: Long
or
@ManyToOne
var `class`: Class
)
@Entity
class Class(
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
var id: Long,
@OneToMany(mappedBy = "classId")
var students: MutableSet<Student> = mutableSetOf()
or
@OneToMany(mappedBy = "class")
var students: MutableSet<Student> = mutableSetOf()
)
|
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| create table class (
id bigint generated by default as identity,
primary key (id)
)
create table student (
id bigint generated by default as identity,
name varchar(255),
class_id bigint not null,
primary key (id)
)
|
![mapped-by]()
Query 비교
INESRT - JOIN TABLE
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| val `class` = classRepository.save(Class())
val students = studentRepository.saveAll(
mutableListOf(
Student(name = "a"),
Student(name = "b"),
Student(name = "c"),
)
).toMutableSet()
`class`.students = students
|
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| Hibernate: insert into class (id) values (default)
Hibernate: insert into student (id, name) values (default, ?)
Hibernate: insert into student (id, name) values (default, ?)
Hibernate: insert into student (id, name) values (default, ?)
Hibernate: insert into class_students (class_id, students_id) values (?, ?)
Hibernate: insert into class_students (class_id, students_id) values (?, ?)
Hibernate: insert into class_students (class_id, students_id) values (?, ?)
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INESRT - MappedBy
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| val `class` = classRepository.save(Class())
val students = studentRepository.saveAll(
mutableListOf(
Student(name = "a", `class` = `class`),
Student(name = "b", `class` = `class`),
Student(name = "c", `class` = `class`),
)
).toMutableSet()
`class`.students = students
|
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| Hibernate: insert into class (id) values (default)
Hibernate: insert into student (id, class_id, name) values (default, ?, ?)
Hibernate: insert into student (id, class_id, name) values (default, ?, ?)
Hibernate: insert into student (id, class_id, name) values (default, ?, ?)
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FetchType은 따로 지정하지않고 기본값(LAZY)으로 비교진행
SELECT - JOIN TABLE
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| Hibernate: select class0_.id as id1_0_0_ from class class0_ where class0_.id=?
Hibernate: select students0_.class_id as class_id1_1_0_, students0_.students_id as students2_1_0_, student1_.id as id1_2_1_, student1_.name as name2_2_1_ from class_students students0_ inner join student student1_ on students0_.students_id=student1_.id where students0_.class_id=?
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SELECT - MappedBy
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| Hibernate: select class0_.id as id1_0_0_ from class class0_ where class0_.id=?
Hibernate: select students0_.class_id as class_id3_1_0_, students0_.id as id1_1_0_, students0_.id as id1_1_1_, students0_.class_id as class_id3_1_1_, students0_.name as name2_1_1_ from student students0_ where students0_.class_id=?
|
