web进修之—Hibernate 关系映射（3）

概述

Hibernate的关系映射是Hibernate使用的难点或者是重点（别担心，不考试哦~），按照不同的分类方式可以对这些映射关系做一个分类，如：

按对象对应关系分：

一对一
多对一/一对多
多对多

按对象对应关系的方向分：

单向
双向

按是否使用连接表分（当然了像多对多是必须使用连接表的）：

使用连接表
不使用连接表

在使用Hibernate中常常不会分得这么仔细，常常是集中分类方式糅合起来使用，但是这个分类可以帮助我们理解Hibernate中的映射关系。

在进行详细的介绍之前，首先需要明确几个点：

Hibernate纷繁复杂的映射关系只是一种面向对象的思维方式，在数据库中就是体现在外键上关联上，所以理解了数据库中的外键关联，再回过头来看映射关系就柳暗花明了；
index在数据库中的作用：为了提高查询该列的效率；
constrain的作用：对该列的值进行约束（废话。。。），各个取值的含义如下

MUL：该列的值可以重复

UNI：该列不能有重复的值

FK：外键，一般定义的时候会有参考哪一个表的哪一个字段

代码

废话少说，是时候展现真正的代码啦

先看两个类Person,Address（你没看错，这是官方用来举例子的两个类）

public class Address {

private long id;

private Set<Person> people;

private Person person;

// setter,getter

}

public class Person {

private long id;

private Address address;

private Set<Address> addressList;

// setter,getter

}

public class Address {

    private long id;

    private Set<Person> people;

    private Person person;
// setter,getter

}

public class Person {

    private long id;

    private Address address;

    private Set<Address> addressList;
// setter,getter

}

各种映射关系的配置文件person.hbm.xml，里面包括单向关联的配置（在靠前面），包含双向关联的配置（紧随单向之后）

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<!DOCTYPE hibernate-mapping PUBLIC

"-//Hibernate/Hibernate Mapping DTD 3.0//EN"

"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">

<hibernate-mapping package="org.lep.hibernate.model" >

<class name="Person" table="person">

<id name="id" column="person_id">

<generator class="increment">

</generator>

</id>

<set name="addressList">

<key column="person_id" >

</key>

<one-to-many class="Address"></one-to-many>

</set>

</class>

</hibernate-mapping>

<!DOCTYPE hibernate-mapping PUBLIC

        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"

        "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="org.lep.hibernate.model" >
<class name="Person" table="person">

        <id name="id" column="person_id">

            <generator class="increment">

            </generator>

        </id>

        <!--单向关联 start-->

        <!--多对一是一对多的反向关联，一对多、多对一就构成了多对多-->
<!--多对一，只需要在many的一方使用many-to-one进行映射即可，和其他id、property一样name表示model里面的属性名称-->

        <!--<many-to-one name="address" column="address_id" not-null="true" />-->
<!--一对一,基于外键，把many-to-one的unique设置为true之后就是单向一对一，在数据库中的体现就是这个外键的值是唯一（key=UNI,如果没有设置unique的话key=mul）的，那也就是只能对应一个Address-->

        <!--有not-null约束的时候注意save对象的顺序，如果没有不用注意顺序hibernate会在保存了关联的对象之后，update到当前对象对应的表-->

        <!--<many-to-one name="address" column="address_id" not-null="true" unique="true" />-->
<!--一对一,基于主键，将主键作为外键的时候，关联的对象和本对象的save顺序是有关系的，要先savereference的对象，然后在保存本对象-->

        <!-- constrained说明主键上存在一个约束，即外键，参考address-->

        <!--<id name="id" column="person_id">-->

            <!--<generator class="foreign">-->

                <!--<param name="property">address</param>-->

            <!--</generator>-->

        <!--</id>-->

        <!--<one-to-one name="address" constrained="true" />-->
<!--一对多-->

        <set name="addressList">

            <key column="person_id" >

            </key>

            <one-to-many class="Address"></one-to-many>

        </set>
<!--单向关联 end-->
<!--单向关联——基于连接表 start-->

        <!--使用多对多和join的时候会产生中间表-->

        <!--一对多使用many-to-many会产生一张中间表-->

        <!--<set name="addressList" table="per_addr">-->

            <!--<key column="person_id"></key>-->

            <!--<many-to-many column="address_id" class="Address" unique="true" />-->

        <!--</set>-->
<!--多对一-->

        <!--<join table="per_addr" optional="true">-->

            <!--<key column="peron_id"></key>-->

            <!--<many-to-one name="address" column="address_id" not-null="true"></many-to-one>-->

        <!--</join>-->
<!--一对一-->

        <!--<join table="per_addr" optional="true">-->

            <!--<key column="person_id"></key>-->

            <!--<many-to-one name="address" column="address_id" not-null="true" unique="true" />-->

        <!--</join>-->
<!--多对多-->

        <!--<set name="addressList" table="per_addr">-->

            <!--<key column="person_id"></key>-->

            <!--<many-to-many column="address_id" class="Address"></many-to-many>-->

        <!--</set>-->

        <!--单向关联——基于连接表 end-->
<!--双向关联 start-->
<!--多对一/一对多-->

        <!--<many-to-one name="address" column="address_id" not-null="true" />-->
<!--一对一，基于外键关联-->

        <!--<many-to-one name="address" column="address_id" unique="true" not-null="true" />-->
<!--一对一，基于主键关联-->

        <!--<one-to-one name="address" />-->
<!--双向关联 end-->
<!--双向关联，使用连接表 start-->
<!--多对一/一对多-->

        <!--<set name="addressList" table="per_addr">-->

            <!--<key column="person_id"></key>-->

            <!--<many-to-many class="Address" column="address_id" unique="true" />-->

        <!--</set>-->
<!--一对一，必须指定column-->

        <!--<join table="per_addr" optional="true">-->

            <!--<key column="person_id" unique="true"></key>-->

            <!--<many-to-one name="address" column="address_id" class="Address" unique="true" />-->

        <!--</join>-->
<!--多对多-->

        <!--<set name="addressList" table="per_addr">-->

        <!--<key column="person_id"></key>-->

        <!--<many-to-many column="address_id" class="Address"></many-to-many>-->

        <!--</set>-->
<!--双向关联，使用连接表 end-->

    </class>

</hibernate-mapping>

address.hbm.xml（只有双向关联的配置）

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<!DOCTYPE hibernate-mapping PUBLIC

"-//Hibernate/Hibernate Mapping DTD 3.0//EN"

"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">

<hibernate-mapping package="org.lep.hibernate.model" >

<class name="Address" table="address">

<id name="id" column="address_id">

<generator class="increment"/>

</id>

</class>

</hibernate-mapping>

<!DOCTYPE hibernate-mapping PUBLIC

        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"

        "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="org.lep.hibernate.model" >

    <class name="Address" table="address">

        <id name="id" column="address_id">

            <generator class="increment"/>

        </id>
<!--双向关联 start-->
<!--多对一/一对多-->

        <!--<set name="people" inverse="true">-->

            <!--<key column="address_id"></key>-->

            <!--<one-to-many class="Person" />-->

        <!--</set>-->
<!--一对一，基于外键关联-->

        <!--<one-to-one name="person" constrained="true"/>-->
<!--一对一，基于主键关联-->

        <!--<id name="id" column="person_id">-->

            <!--<generator class="foreign">-->

                <!--<param name="property">person</param>-->

            <!--</generator>-->

        <!--</id>-->

        <!--<one-to-one name="person" constrained="true"/>-->
<!--双向关联 end-->
<!--双向关联，使用连接表 start-->
<!--多对一/一对多-->

        <!--<join table="per_addr" inverse="true" optional="true">-->

            <!--<key column="address_id"></key>-->

            <!--<many-to-one name="person" column="person_id" not-null="true"></many-to-one>-->

        <!--</join>-->
<!--一对一，必须指定column-->

        <!--<join table="per_addr" inverse="true" optional="true">-->

            <!--<key column="address_id" unique="true"></key>-->

            <!--<many-to-one name="person" column="person_id" unique="true"  />-->

        <!--</join>-->
<!--多对多-->

        <!--<set name="people" inverse="true" table="per_addr">-->

        <!--<key column="address_id"></key>-->

        <!--<many-to-many column="person_id" class="Person"></many-to-many>-->

        <!--</set>-->
<!--双向关联，使用连接表 end-->
</class>

</hibernate-mapping>

试验过这些配置之后，我们可以总结出：

一对一可以通过主键关联，也可以使用外键关联
通过使用unique属性可以将多（many）的关系变为一（one）的关系，比如一对一，可以使用many-to-one unique=”true”
在使用many-to-many或者使用join的时候才会产生连接表（中间表），可以根据实际情况来决定是否采用产生中间表的配置（比如查询的性能，业务逻辑的需要）
如果是双向关系，在一边配置之后，另一边使用inverse=”true”来告诉Hibernate有谁来控制
推荐外键设置not null，这样设置后就要注意保存对象的顺序了，比如person对address单向关联（Person有一个属性Address），那么person表里面就会有一个address_id的外键，如果这个外键设为not null，那么save的时候应该先save address，再save person

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Person p = new Person();

Address addr = new Address();

p.setAddress(addr);

// 正确

session.beginTransaction();

session.save(address);

session.save(p);

session.getTransaction().commit();

// 运行会报错，因为外键address设为了not null，在save p的时候，address_id还没有值，所以不满足非空的约束

session.beginTransaction();

session.save(p);

session.save(address);

session.getTransaction().commit();

Person p = new Person();

Address addr = new Address();

p.setAddress(addr);
// 正确

session.beginTransaction();

session.save(address);

session.save(p);

session.getTransaction().commit();
// 运行会报错，因为外键address设为了not null，在save p的时候，address_id还没有值，所以不满足非空的约束

session.beginTransaction();

session.save(p);

session.save(address);

session.getTransaction().commit();

因为外键address设为了not null，在save p的时候，address_id还没有值，所以不满足非空的约束

如果说大家对Hibernate理解比较熟了，只是想看看具体配置文件怎么样写，那么到上面就可以了，如果还想探究一下Hibernate和数据库的对应关系那请继续，先看一个文件（HIbernate在建立上面的映射关系的时候产生的数据库语句）

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单向

多对一

Hibernate: create table address (address_id bigint not null, primary key (address_id))

Hibernate: create table person (person_id bigint not null, address_id bigint not null, primary key (person_id))

Hibernate: alter table person add index FK_o8tnkglv9n1eeqmo7de7em37n (address_id), add constraint FK_o8tnkglv9n1eeqmo7de7em37n foreign key (address_id) references address (address_id)

一对一

基于外键

Hibernate: create table address (address_id bigint not null, primary key (address_id))

Hibernate: create table person (person_id bigint not null, address_id bigint not null, primary key (person_id))

Hibernate: alter table person add constraint UK_o8tnkglv9n1eeqmo7de7em37n unique (address_id)

Hibernate: alter table person add index FK_o8tnkglv9n1eeqmo7de7em37n (address_id), add constraint FK_o8tnkglv9n1eeqmo7de7em37n foreign key (address_id) references address (address_id)

基于主键

Hibernate: create table address (address_id bigint not null, primary key (address_id))

Hibernate: create table person (person_id bigint not null, primary key (person_id))

Hibernate: alter table person add index FK_acrq16tm1ioc620qk2nm5gwyg (person_id), add constraint FK_acrq16tm1ioc620qk2nm5gwyg foreign key (person_id) references address (address_id)

一对多

Hibernate: create table address (address_id bigint not null, person_id bigint, primary key (address_id))

Hibernate: create table person (person_id bigint not null, primary key (person_id))

Hibernate: alter table address add index FK_5k57pkctki2o1wpmk2880r74j (person_id), add constraint FK_5k57pkctki2o1wpmk2880r74j foreign key (person_id) references person (person_id)

单向——中间表

一对多

Hibernate: create table address (address_id bigint not null, primary key (address_id))

Hibernate: create table per_addr (person_id bigint not null, address_id bigint not null, primary key (person_id, address_id))

Hibernate: create table person (person_id bigint not null, primary key (person_id))

Hibernate: alter table per_addr add constraint UK_8v3twe5k7nlb8wcjqvcpydab6 unique (address_id)

Hibernate: alter table per_addr add index FK_8v3twe5k7nlb8wcjqvcpydab6 (address_id), add constraint FK_8v3twe5k7nlb8wcjqvcpydab6 foreign key (address_id) references address (address_id)

Hibernate: alter table per_addr add index FK_hdwakolgq6oelbfuallvfbcn4 (person_id), add constraint FK_hdwakolgq6oelbfuallvfbcn4 foreign key (person_id) references person (person_id)

多对一

Hibernate: create table address (address_id bigint not null, primary key (address_id))

Hibernate: create table per_addr (peron_id bigint not null, address_id bigint not null, primary key (peron_id))

Hibernate: create table person (person_id bigint not null, primary key (person_id))

Hibernate: alter table per_addr add index FK_7xqe3kidwvogwcohihqla5ehv (peron_id), add constraint FK_7xqe3kidwvogwcohihqla5ehv foreign key (peron_id) references person (person_id)

Hibernate: alter table per_addr add index FK_8v3twe5k7nlb8wcjqvcpydab6 (address_id), add constraint FK_8v3twe5k7nlb8wcjqvcpydab6 foreign key (address_id) references address (address_id)

一对一

Hibernate: create table address (address_id bigint not null, primary key (address_id))

Hibernate: create table per_addr (person_id bigint not null, address_id bigint not null, primary key (person_id))

Hibernate: create table person (person_id bigint not null, primary key (person_id))

Hibernate: alter table per_addr add constraint UK_8v3twe5k7nlb8wcjqvcpydab6 unique (address_id)

Hibernate: alter table per_addr add index FK_hdwakolgq6oelbfuallvfbcn4 (person_id), add constraint FK_hdwakolgq6oelbfuallvfbcn4 foreign key (person_id) references person (person_id)

Hibernate: alter table per_addr add index FK_8v3twe5k7nlb8wcjqvcpydab6 (address_id), add constraint FK_8v3twe5k7nlb8wcjqvcpydab6 foreign key (address_id) references address (address_id)

多对多

Hibernate: create table address (address_id bigint not null, primary key (address_id))

Hibernate: create table per_addr (person_id bigint not null, address_id bigint not null, primary key (person_id, address_id))

Hibernate: create table person (person_id bigint not null, primary key (person_id))

Hibernate: alter table per_addr add index FK_8v3twe5k7nlb8wcjqvcpydab6 (address_id), add constraint FK_8v3twe5k7nlb8wcjqvcpydab6 foreign key (address_id) references address (address_id)

Hibernate: alter table per_addr add index FK_hdwakolgq6oelbfuallvfbcn4 (person_id), add constraint FK_hdwakolgq6oelbfuallvfbcn4 foreign key (person_id) references person (person_id)

双向

多对一/一对多

Hibernate: create table address (address_id bigint not null, primary key (address_id))

Hibernate: create table person (person_id bigint not null, address_id bigint not null, primary key (person_id))

Hibernate: alter table person add index FK_o8tnkglv9n1eeqmo7de7em37n (address_id), add constraint FK_o8tnkglv9n1eeqmo7de7em37n foreign key (address_id) references address (address_id)

一对一

基于外键

Hibernate: create table address (address_id bigint not null, primary key (address_id))

Hibernate: create table person (person_id bigint not null, address_id bigint not null, primary key (person_id))

Hibernate: alter table person add constraint UK_o8tnkglv9n1eeqmo7de7em37n unique (address_id)

Hibernate: alter table address add index FK_scpdoha0q1mmbp5f9lojr3s9x (address_id), add constraint FK_scpdoha0q1mmbp5f9lojr3s9x foreign key (address_id) references person (person_id)

Hibernate: alter table person add index FK_o8tnkglv9n1eeqmo7de7em37n (address_id), add constraint FK_o8tnkglv9n1eeqmo7de7em37n foreign key (address_id) references address (address_id)

基于主键

Hibernate: create table address (person_id bigint not null, primary key (person_id))

Hibernate: create table person (person_id bigint not null, primary key (person_id))

Hibernate: alter table address add index FK_5k57pkctki2o1wpmk2880r74j (person_id), add constraint FK_5k57pkctki2o1wpmk2880r74j foreign key (person_id) references person (person_id)

双向——中间表

多对一/一对多

Hibernate: create table address (address_id bigint not null, primary key (address_id))

Hibernate: create table per_addr (address_id bigint not null, person_id bigint not null, primary key (person_id, address_id))

Hibernate: create table person (person_id bigint not null, primary key (person_id))

Hibernate: alter table per_addr add index FK_8v3twe5k7nlb8wcjqvcpydab6 (address_id), add constraint FK_8v3twe5k7nlb8wcjqvcpydab6 foreign key (address_id) references address (address_id)

Hibernate: alter table per_addr add index FK_hdwakolgq6oelbfuallvfbcn4 (person_id), add constraint FK_hdwakolgq6oelbfuallvfbcn4 foreign key (person_id) references person (person_id)

一对一

Hibernate: create table address (address_id bigint not null, primary key (address_id))

Hibernate: create table per_addr (person_id bigint not null, address_id bigint, primary key (address_id))

Hibernate: create table person (person_id bigint not null, primary key (person_id))

Hibernate: alter table per_addr add constraint UK_hdwakolgq6oelbfuallvfbcn4 unique (person_id)

Hibernate: alter table per_addr add index FK_hdwakolgq6oelbfuallvfbcn4 (person_id), add constraint FK_hdwakolgq6oelbfuallvfbcn4 foreign key (person_id) references person (person_id)

Hibernate: alter table per_addr add index FK_8v3twe5k7nlb8wcjqvcpydab6 (address_id), add constraint FK_8v3twe5k7nlb8wcjqvcpydab6 foreign key (address_id) references address (address_id)

多对多

Hibernate: create table address (address_id bigint not null, primary key (address_id))

Hibernate: create table per_addr (person_id bigint not null, address_id bigint not null, primary key (person_id, address_id))

Hibernate: create table person (person_id bigint not null, primary key (person_id))

Hibernate: alter table per_addr add index FK_8v3twe5k7nlb8wcjqvcpydab6 (address_id), add constraint FK_8v3twe5k7nlb8wcjqvcpydab6 foreign key (address_id) references address (address_id)

Hibernate: alter table per_addr add index FK_hdwakolgq6oelbfuallvfbcn4 (person_id), add constraint FK_hdwakolgq6oelbfuallvfbcn4 foreign key (person_id) references person (person_id)

单向

多对一

Hibernate: create table address (address_id bigint not null, primary key (address_id))

Hibernate: create table person (person_id bigint not null, address_id bigint not null, primary key (person_id))

Hibernate: alter table person add index FK_o8tnkglv9n1eeqmo7de7em37n (address_id), add constraint FK_o8tnkglv9n1eeqmo7de7em37n foreign key (address_id) references address (address_id)
一对一

    基于外键

Hibernate: create table address (address_id bigint not null, primary key (address_id))

Hibernate: create table person (person_id bigint not null, address_id bigint not null, primary key (person_id))

Hibernate: alter table person add constraint UK_o8tnkglv9n1eeqmo7de7em37n unique (address_id)

Hibernate: alter table person add index FK_o8tnkglv9n1eeqmo7de7em37n (address_id), add constraint FK_o8tnkglv9n1eeqmo7de7em37n foreign key (address_id) references address (address_id)
基于主键

Hibernate: create table address (address_id bigint not null, primary key (address_id))

Hibernate: create table person (person_id bigint not null, primary key (person_id))

Hibernate: alter table person add index FK_acrq16tm1ioc620qk2nm5gwyg (person_id), add constraint FK_acrq16tm1ioc620qk2nm5gwyg foreign key (person_id) references address (address_id)
一对多

Hibernate: create table address (address_id bigint not null, person_id bigint, primary key (address_id))

Hibernate: create table person (person_id bigint not null, primary key (person_id))

Hibernate: alter table address add index FK_5k57pkctki2o1wpmk2880r74j (person_id), add constraint FK_5k57pkctki2o1wpmk2880r74j foreign key (person_id) references person (person_id)
单向——中间表

一对多

Hibernate: create table address (address_id bigint not null, primary key (address_id))

Hibernate: create table per_addr (person_id bigint not null, address_id bigint not null, primary key (person_id, address_id))

Hibernate: create table person (person_id bigint not null, primary key (person_id))

Hibernate: alter table per_addr add constraint UK_8v3twe5k7nlb8wcjqvcpydab6 unique (address_id)

Hibernate: alter table per_addr add index FK_8v3twe5k7nlb8wcjqvcpydab6 (address_id), add constraint FK_8v3twe5k7nlb8wcjqvcpydab6 foreign key (address_id) references address (address_id)

Hibernate: alter table per_addr add index FK_hdwakolgq6oelbfuallvfbcn4 (person_id), add constraint FK_hdwakolgq6oelbfuallvfbcn4 foreign key (person_id) references person (person_id)
多对一

Hibernate: create table address (address_id bigint not null, primary key (address_id))

Hibernate: create table per_addr (peron_id bigint not null, address_id bigint not null, primary key (peron_id))

Hibernate: create table person (person_id bigint not null, primary key (person_id))

Hibernate: alter table per_addr add index FK_7xqe3kidwvogwcohihqla5ehv (peron_id), add constraint FK_7xqe3kidwvogwcohihqla5ehv foreign key (peron_id) references person (person_id)

Hibernate: alter table per_addr add index FK_8v3twe5k7nlb8wcjqvcpydab6 (address_id), add constraint FK_8v3twe5k7nlb8wcjqvcpydab6 foreign key (address_id) references address (address_id)
一对一

Hibernate: create table address (address_id bigint not null, primary key (address_id))

Hibernate: create table per_addr (person_id bigint not null, address_id bigint not null, primary key (person_id))

Hibernate: create table person (person_id bigint not null, primary key (person_id))

Hibernate: alter table per_addr add constraint UK_8v3twe5k7nlb8wcjqvcpydab6 unique (address_id)

Hibernate: alter table per_addr add index FK_hdwakolgq6oelbfuallvfbcn4 (person_id), add constraint FK_hdwakolgq6oelbfuallvfbcn4 foreign key (person_id) references person (person_id)

Hibernate: alter table per_addr add index FK_8v3twe5k7nlb8wcjqvcpydab6 (address_id), add constraint FK_8v3twe5k7nlb8wcjqvcpydab6 foreign key (address_id) references address (address_id)
多对多

Hibernate: create table address (address_id bigint not null, primary key (address_id))

Hibernate: create table per_addr (person_id bigint not null, address_id bigint not null, primary key (person_id, address_id))

Hibernate: create table person (person_id bigint not null, primary key (person_id))

Hibernate: alter table per_addr add index FK_8v3twe5k7nlb8wcjqvcpydab6 (address_id), add constraint FK_8v3twe5k7nlb8wcjqvcpydab6 foreign key (address_id) references address (address_id)

Hibernate: alter table per_addr add index FK_hdwakolgq6oelbfuallvfbcn4 (person_id), add constraint FK_hdwakolgq6oelbfuallvfbcn4 foreign key (person_id) references person (person_id)
双向

多对一/一对多

Hibernate: create table address (address_id bigint not null, primary key (address_id))

Hibernate: create table person (person_id bigint not null, address_id bigint not null, primary key (person_id))

Hibernate: alter table person add index FK_o8tnkglv9n1eeqmo7de7em37n (address_id), add constraint FK_o8tnkglv9n1eeqmo7de7em37n foreign key (address_id) references address (address_id)

一对一

    基于外键

Hibernate: create table address (address_id bigint not null, primary key (address_id))

Hibernate: create table person (person_id bigint not null, address_id bigint not null, primary key (person_id))

Hibernate: alter table person add constraint UK_o8tnkglv9n1eeqmo7de7em37n unique (address_id)

Hibernate: alter table address add index FK_scpdoha0q1mmbp5f9lojr3s9x (address_id), add constraint FK_scpdoha0q1mmbp5f9lojr3s9x foreign key (address_id) references person (person_id)

Hibernate: alter table person add index FK_o8tnkglv9n1eeqmo7de7em37n (address_id), add constraint FK_o8tnkglv9n1eeqmo7de7em37n foreign key (address_id) references address (address_id)
基于主键

Hibernate: create table address (person_id bigint not null, primary key (person_id))

Hibernate: create table person (person_id bigint not null, primary key (person_id))

Hibernate: alter table address add index FK_5k57pkctki2o1wpmk2880r74j (person_id), add constraint FK_5k57pkctki2o1wpmk2880r74j foreign key (person_id) references person (person_id)
双向——中间表

多对一/一对多

Hibernate: create table address (address_id bigint not null, primary key (address_id))

Hibernate: create table per_addr (address_id bigint not null, person_id bigint not null, primary key (person_id, address_id))

Hibernate: create table person (person_id bigint not null, primary key (person_id))

Hibernate: alter table per_addr add index FK_8v3twe5k7nlb8wcjqvcpydab6 (address_id), add constraint FK_8v3twe5k7nlb8wcjqvcpydab6 foreign key (address_id) references address (address_id)

Hibernate: alter table per_addr add index FK_hdwakolgq6oelbfuallvfbcn4 (person_id), add constraint FK_hdwakolgq6oelbfuallvfbcn4 foreign key (person_id) references person (person_id)
一对一

Hibernate: create table address (address_id bigint not null, primary key (address_id))

Hibernate: create table per_addr (person_id bigint not null, address_id bigint, primary key (address_id))

Hibernate: create table person (person_id bigint not null, primary key (person_id))

Hibernate: alter table per_addr add constraint UK_hdwakolgq6oelbfuallvfbcn4 unique (person_id)

Hibernate: alter table per_addr add index FK_hdwakolgq6oelbfuallvfbcn4 (person_id), add constraint FK_hdwakolgq6oelbfuallvfbcn4 foreign key (person_id) references person (person_id)

Hibernate: alter table per_addr add index FK_8v3twe5k7nlb8wcjqvcpydab6 (address_id), add constraint FK_8v3twe5k7nlb8wcjqvcpydab6 foreign key (address_id) references address (address_id)
多对多

Hibernate: create table address (address_id bigint not null, primary key (address_id))

Hibernate: create table per_addr (person_id bigint not null, address_id bigint not null, primary key (person_id, address_id))

Hibernate: create table person (person_id bigint not null, primary key (person_id))

Hibernate: alter table per_addr add index FK_8v3twe5k7nlb8wcjqvcpydab6 (address_id), add constraint FK_8v3twe5k7nlb8wcjqvcpydab6 foreign key (address_id) references address (address_id)

Hibernate: alter table per_addr add index FK_hdwakolgq6oelbfuallvfbcn4 (person_id), add constraint FK_hdwakolgq6oelbfuallvfbcn4 foreign key (person_id) references person (person_id)

通过上面的SQL语句，我们可以得出以下几个结论

单向、双向是在对象层面上去考虑的，其实在数据库层面上是一致的，比如单向多对一和双向多对一在表结构上一样的
为了提高效率一般在（中间表）外键上都会创建index

Hibernate中的关系映射是最常用、也是最复杂的。在之前的点滴学习中对Hibernate的映射关系也是一知半解，这次特地花时间认真整理学习了一下，特别是对Hibernate和数据库中表的对应关系进行了认真的探究。

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