JPA 一对多关系是指一个实体对象可以有多个关联实体对象，而这些关联实体对象只能有一个实体对象作为其关系的拥有者。JPA 一对多关系是 JPA 中最常用的关系之一，在大多数情况下，我们都会需要使用这种关系来表示数据库中的表之间的关系。
JPA 一对多关系可以通过 @OneToMany 来表示，@OneToMany 是 JPA 的注释，用于标识实体之间的一对多的映射。@OneToMany 是 JPA 生成数据库表之间的映射时非常重要的注释，因此在使用 JPA 时必须正确使用 @OneToMany 来标识实体之间的映射。
@Entity
public class Department {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
private String name;
@OneToMany(mappedBy = "department")
private List<Employee> employees;
// getter and setter methods...
}以下代码显示如何在一对多映射中按属性设置顺序。
@OrderBy(“name ASC")设置按人员名称设置的人员排序。
@OneToMany(targetEntity=Person.class, mappedBy="department")
@OrderBy("name ASC")
private Set<Person> persons = new HashSet<Person>();
下面的代码来自PersonDaoImpl.java。
package cn..common;
import java.util.List;
import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;
import javax.persistence.Query;
import org.springframework.transaction.annotation.Transactional;
@Transactional
public class PersonDaoImpl {
public void test(){
Department d = new Department();
d.setName("Design");
Person p1 = new Person("Tom");
p1.setDepartment(d);
Person p2 = new Person("Jack");
p2.setDepartment(d);
d.getPersons().add(p1);
d.getPersons().add(p2);
em.persist(p1);
em.persist(p2);
em.persist(d);
}
@PersistenceContext
private EntityManager em;
}
下面的代码来自Department.java。
package cn..common;
import java.util.HashSet;
import java.util.Set;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.OneToMany;
import javax.persistence.OrderBy;
@Entity
public class Department {
@Id
@GeneratedValue(strategy=GenerationType.IDENTITY)
private long id;
private String name;
@OneToMany(targetEntity=Person.class, mappedBy="department")
@OrderBy("name ASC")
private Set<Person> persons = new HashSet<Person>();
public Set<Person> getPersons() {
return persons;
}
public void setPersons(Set<Person> persons) {
this.persons = persons;
}
public long getId() {
return id;
}
public void setId(long id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
@Override
public String toString() {
return "Department [id=" + id + ", name=" + name + ", persons=" + persons
+ "]";
}
}
下面的代码来自Person.java。
package cn..common;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.ManyToOne;
import javax.persistence.CascadeType;
@Entity
public class Person {
@Id
@GeneratedValue(strategy=GenerationType.IDENTITY)
private long id;
private String name;
@ManyToOne (cascade=CascadeType.ALL)
private Department department;
public Person() {}
public Person(String name) {
this.name = name;
}
public Department getDepartment() {
return department;
}
public void setDepartment(Department department) {
this.department = department;
}
public Long getId() {
return id;
}
public void setId(Long id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
@Override
public String toString() {
return "Person [id=" + id + ", name=" + name + "]";
}
}
下载 OneToMany_OrderBy.zip以下是数据库转储。
Table Name: DEPARTMENT
Row:
Column Name: ID,
Column Type: BIGINT:
Column Value: 1
Column Name: NAME,
Column Type: VARCHAR:
Column Value: Design
Table Name: PERSON
Row:
Column Name: ID,
Column Type: BIGINT:
Column Value: 1
Column Name: NAME,
Column Type: VARCHAR:
Column Value: Tom
Column Name: DEPARTMENT_ID,
Column Type: BIGINT:
Column Value: 1
Row:
Column Name: ID,
Column Type: BIGINT:
Column Value: 2
Column Name: NAME,
Column Type: VARCHAR:
Column Value: Jack
Column Name: DEPARTMENT_ID,
Column Type: BIGINT:
Column Value: 1
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