jpa单向一对多关联映射

如果在一的@OneToMany有@manyToOne则是双向一对多关联，如果在多的那面没有@manyToOne关联则是单向一对多关联

class和student是一对多的关系

表结构

student

class

Class

package auth.model;

import java.util.ArrayList;
import java.util.List;

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.OneToMany;
import javax.persistence.Table;

@Entity
@Table(name="class")
public class Room {
	private int id;
	private String name;
	private List<Student> students=new ArrayList<Student>();
	@Id
	@GeneratedValue(strategy= GenerationType.AUTO)
	@Column(name="id")
	public int getId() {
		return id;
	}
	public void setId(int id) {
		this.id = id;
	}
	@Column(name="name")
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	@OneToMany(cascade = CascadeType.ALL,fetch=FetchType.EAGER) 
　　　　@JoinColumn(name="room_id") 
　　　　public List<Student> getStudents() {
　　　　　　 return students; 
　　　　} 
　　　　public void setStudents(List<Student> students) {
　　　　 　　this.students = students; 
　　　　}
 }

Student

package auth.model;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.Table;

@Entity
@Table(name="student")
public class Student {
	private int id;
	public int room_id;
	private String name;
	@GeneratedValue(strategy= GenerationType.AUTO)
	@Id
	@Column(name="id")
	public int getId() {
		return id;
	}
	public void setId(int id) {
		this.id = id;
	}
	@Column(name="room_id")
	public int getRoom_id() {
		return room_id;
	}
	public void setRoom_id(int room_id) {
		this.room_id= room_id;
	}

	@Column(name="name")
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}

}

Dao

package auth.dao;

import org.springframework.data.jpa.repository.JpaRepository;

import auth.model.MyClass;

public interface ClassDao extends JpaRepository<MyClass, Integer>{

}

测试

@Autowired
	private ClassDao dao;
	public void add(){
		Room room=new Room();
		room.setName("5班");
		Student s=new Student();
		s.setName("小王");
		room.getStudents().add(s);
		dao.save(room);
		Student s1=new Student();
		s.setName("小李");
		room.getStudents().add(s1);
		dao.save(room);
	}

public void query(){
		Student s=stuDao.findOne(2);
		System.out.println(JSONObject.toJSONString(s));

	}
	public void query1(){
		Rooms=dao.findOne(2);
		System.out.println(JSONObject.toJSONString(s));

	}

过程

insert
    into
        class
        (name)
    values
        (?)
2017-06-04 23:56:04,251 DEBUG [org.hibernate.id.IdentifierGeneratorHelper:94] - Natively generated identity: 3
2017-06-04 23:56:04,272 DEBUG [org.hibernate.engine.spi.ActionQueue:196] - Executing identity-insert immediately
2017-06-04 23:56:04,273 DEBUG [org.hibernate.SQL:109] -
    insert
    into
        student
        (room_id, name)
    values
        (?, ?)

第一个查询只能查出Student的数据，第二个可以把Class和Student的数据查出来（要设置fetch=FetchType.EAGER）