one-to-one外鍵雙向關聯之建表

首先是兩個類,在中國是實行一夫一妻，也就是一個husband只能對應一個wife

Wife類，產生的表為t_wife

/** *  */package com.maybe.test_1;import javax.persistence.Entity;import javax.persistence.GeneratedValue;import javax.persistence.Id;import javax.persistence.OneToOne;import javax.persistence.Table;/** * @author MayBe * * function: */@Entity@Table(name="t_wife")public class Wife {private Integer id;private String name;private Husband husband;@Id@GeneratedValuepublic Integer getId() {return id;}public String getName() {return name;}public void setId(Integer id) {this.id = id;}public void setName(String name) {this.name = name;}@OneToOnepublic Husband getHusband() {return husband;}public void setHusband(Husband husband) {this.husband = husband;}}

Husband類，產生的表為t_husband

/** *  */package com.maybe.test_1;import javax.persistence.Entity;import javax.persistence.GeneratedValue;import javax.persistence.Id;import javax.persistence.JoinColumn;import javax.persistence.OneToOne;import javax.persistence.Table;/** * @author MayBe * * function: */@Entity@Table(name="t_husband")public class Husband {private Integer id;private String name;private Wife wife;@Id@GeneratedValuepublic Integer getId() {return id;}public String getName() {return name;}public void setId(Integer id) {this.id = id;}public void setName(String name) {this.name = name;}@OneToOnepublic Wife getWife() {return wife;}public void setWife(Wife wife) {this.wife = wife;}}

因為是一對一雙向關聯，wife類和husband類都包含對方的引用

hibernate.cfg.xml檔案如下：

<?xml version='1.0' encoding='utf-8'?><!DOCTYPE hibernate-configuration PUBLIC        "-//Hibernate/Hibernate Configuration DTD 3.0//EN"        "http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd"><hibernate-configuration>    <session-factory>        <!-- Database connection settings -->        <property name="connection.driver_class">com.microsoft.sqlserver.jdbc.SQLServerDriver</property>        <property name="connection.url">jdbc:sqlserver://localhost;DatabaseName=hibernate</property>        <property name="connection.username">sa</property>        <property name="connection.password">yaphets</property>        <!-- JDBC connection pool (use the built-in) -->        <property name="connection.pool_size">1</property>        <!-- SQL dialect -->        <property name="dialect">org.hibernate.dialect.SQLServerDialect</property>        <!-- Enable Hibernate's automatic session context management -->                  <property name="current_session_context_class">thread</property>              <!-- Disable the second-level cache  -->        <property name="cache.provider_class">org.hibernate.cache.internal.NoCacheProvider</property>        <!-- Echo all executed SQL to stdout -->        <property name="show_sql">true</property>        <!-- Drop and re-create the database schema on startup -->               <property name="hbm2ddl.auto">update</property>                <property name="format_sql">true</property>        <!--          <mapping resource="com/maybe/test_1/Player.hbm.xml"/>                -->        <mapping class="com.maybe.test_1.Husband"/>        <mapping class="com.maybe.test_1.Wife"/>    </session-factory></hibernate-configuration>

用Junit進行測試，產生的建表語句如下所示

drop table t_wife

    create table t_husband (
        id int identity not null,
        name varchar(255),
        wife_id int,
        primary key (id)
    )

    create table t_wife (
        id int identity not null,
        name varchar(255),
        husband_id int,
        primary key (id)
    )

    alter table t_husband
        add constraint FK_hxagmw4p5aym8m63pxd6h2wwx
        foreign key (wife_id)
        references t_wife

    alter table t_wife
        add constraint FK_fi3kodkmubgryyblf4935y4dk
        foreign key (husband_id)
        references t_husband

不難看出這種一對一關聯性比較冗餘，這個表的設計明顯有問題，原因是我們在使用註解的時候One-to-one，是由hibernate自動產生的，解決辦法非常簡單，只要在註解上添加一個屬性就可以，比如Husband類：

/** *  */package com.maybe.test_1;import javax.persistence.Entity;import javax.persistence.GeneratedValue;import javax.persistence.Id;import javax.persistence.JoinColumn;import javax.persistence.OneToOne;import javax.persistence.Table;/** * @author MayBe * * function: */@Entity@Table(name="t_husband")public class Husband {private Integer id;private String name;private Wife wife;@Id@GeneratedValuepublic Integer getId() {return id;}public String getName() {return name;}public void setId(Integer id) {this.id = id;}public void setName(String name) {this.name = name;}@OneToOne(mappedBy="husband")public Wife getWife() {return wife;}public void setWife(Wife wife) {this.wife = wife;}}

我們在onetoone中使用了mappedby屬性(在雙向關聯中一般都要用到),指明了mappedBy ，就指明了這個映射關係的承擔方(Wife類中的husband)，產生的資料表樹下所示：

 create table t_husband (
        id int identity not null,
        name varchar(255),
        primary key (id)
    )

    create table t_wife (
        id int identity not null,
        name varchar(255),
        husband_id int,
        primary key (id)
    )

    alter table t_wife
        add constraint FK_fi3kodkmubgryyblf4935y4dk
        foreign key (husband_id)
        references t_husband

這次產生的表沒有冗餘，在wife表中有一個外鍵husband_id，正是通過husband_id來維護兩個標的聯絡，所以雙向關係中，要加上mappedBy屬性確保資料庫表的正確性