hibernate中一对多关系模式的设计

hibernate中一对多的关系模式的设计

注解应该交由多的一方维护和管理。

例如：bbs的模块设计

三个表 版块表(category) 、主题帖表(topic)和回帖表(msg)

他们之间的对应关系是：一个版块对应多个主题帖，一个主题帖有多个回复贴。

以下是对应的建表语句。

CREATE TABLE `category` (
  `id` int(11) NOT NULL auto_increment,
  `name` varchar(255) default NULL,
  PRIMARY KEY  (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=21 DEFAULT CHARSET=utf8;

CREATE TABLE `topic` (
  `id` int(11) NOT NULL auto_increment,
  `createDate` datetime default NULL,
  `title` varchar(255) default NULL,
  `category_id` int(11) default NULL,
  PRIMARY KEY  (`id`),
  KEY `FK_jqmw7lsw6i6lrdnw07c1id1nt` (`category_id`),
  CONSTRAINT `FK_jqmw7lsw6i6lrdnw07c1id1nt` FOREIGN KEY (`category_id`) REFERENCES `category` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=21 DEFAULT CHARSET=utf8;

CREATE TABLE `msg` (
  `id` int(11) NOT NULL auto_increment,
  `cont` varchar(255) default NULL,
  `topic_id` int(11) default NULL,
  PRIMARY KEY  (`id`),
  KEY `FK_87qfcl8orshwjlllfik5deys5` (`topic_id`),
  CONSTRAINT `FK_87qfcl8orshwjlllfik5deys5` FOREIGN KEY (`topic_id`) REFERENCES `topic` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=21 DEFAULT CHARSET=utf8;

那么在对象模型中是如何设计的呢？

实际过程中是通过hibernate的逆向工程生成类

import java.util.Date;

import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.ManyToOne;

//bbs版块下的主题  多个主题对应一个版块
@Entity
public class Topic {
	private int id;
	private String title;
	private Category category;
	//private Category category2;
	
	private Date createDate;
	
	public Date getCreateDate() {
		return createDate;
	}
	public void setCreateDate(Date createDate) {
		this.createDate = createDate;
	}
	@ManyToOne(fetch=FetchType.LAZY)
	public Category getCategory() {
		return category;
	}
	public void setCategory(Category category) {
		this.category = category;
	}
	@Id
	@GeneratedValue
	public int getId() {
		return id;
	}
	public void setId(int id) {
		this.id = id;
	}
	public String getTitle() {
		return title;
	}
	public void setTitle(String title) {
		this.title = title;
	}
	
}

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;

//bbs的版块 
@Entity
public class Category {
	private int id;
	private String name;
	@Id
	@GeneratedValue
	public int getId() {
		return id;
	}
	public void setId(int id) {
		this.id = id;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
}

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.ManyToOne;

//bbs主题下面的回帖   多个回帖位于一个主题帖之下
@Entity
public class Msg {
	private int id;
	private String cont;
	private Topic topic;
	@ManyToOne
	public Topic getTopic() {
		return topic;
	}
	public void setTopic(Topic topic) {
		this.topic = topic;
	}
	@Id
	@GeneratedValue
	public int getId() {
		return id;
	}
	public void setId(int id) {
		this.id = id;
	}
	
	public String getCont() {
		return cont;
	}
	public void setCont(String cont) {
		this.cont = cont;
	}
}

关于注解的几个注意点：

1.当表名和类名不同,对表名进行配置

@Entity

@Table(name="_teacher")

public class Teacher {

 }

2.当字段名和属性名不同时，对属性名进行配置

@Column(name="_name")

   public String getName() {

    return name;

}

3.manytoone注解交由多的哪一方维护和管理

4.映射日期和时间的类型，指定精度

    @Temporal(参数) 参数有3种 只显示时间,只显示日期,时间日期都显示

       //@Temporal(TemporalType.DATE)  只显示日期

       //@Temporal(TemporalType.TIME)  只显示时间

       //@Temporal(TemporalType.TIMESTAMP)  显示日期与时间