Hibernate

第8章关联关系映射

8.1.介绍

关联关系映射通常情况是最难配置正确的。在这个部分中，我们从单向关系映射开始，然后考虑双向关系映射，由浅至深讲述一遍典型的案例。在所有的例子中，我们都使用Person和Address。

我们根据映射关系是否涉及连接表以及多样性来划分关联类型。

在传统的数据建模中，允许为Null值的外键被认为是一种不好的实践，因此我们所有的例子中都使用不允许为Null的外键。这并不是Hibernate的要求，即使你删除掉不允许为Null的约束，Hibernate映射一样可以工作的很好。

8.2.单向关联（Unidirectional associations）

8.2.1.多对一(many to one)

单向many-to-one关联是最常见的单向关联关系。

<class name="Person">

<id name="id"column="personId">

<generatorclass="native"/>

</id>

<many-to-onename="address"

column="addressId"

not-null="true"/>

</class>

<class name="Address">

<id name="id"column="addressId">

<generator class="native"/>

</id>

</class>

create table Person ( personId bigint not null primary key, addressIdbigint not null )

create table Address ( addressId bigint not null primary key )

8.2.2.一对一（one to one）

基于外键关联的单向一对一关联和单向多对一关联几乎是一样的。唯一的不同就是单向一对一关联中的外键字段具有唯一性约束。

<class name="Person">

<id name="id"column="personId">

<generatorclass="native"/>

</id>

<many-to-onename="address"

column="addressId"

unique="true"

not-null="true"/>

</class>

<class name="Address">

<id name="id"column="addressId">

<generatorclass="native"/>

</id>

</class>

create table Person ( personId bigint not null primary key, addressIdbigint not null unique )

create table Address ( addressId bigint not null primary key )

基于主键关联的单向一对一关联通常使用一个特定的id生成器。（请注意，在这个例子中我们掉换了关联的方向。）

<class name="Person">

<id name="id"column="personId">

<generatorclass="native"/>

</id>

</class>

<class name="Address">

<id name="id"column="personId">

<generatorclass="foreign">

<paramname="property">person</param>

</generator>

</id>

<one-to-onename="person" constrained="true"/>

</class>

create table Person ( personId bigint not null primary key )

create table Address ( personId bigint not null primary key )

8.2.3.一对多（one to many）

基于外键关联的单向一对多关联是一种很少见的情况，并不推荐使用。

<class name="Person">

<id name="id"column="personId">

<generatorclass="native"/>

</id>

<setname="addresses">

<keycolumn="personId"

not-null="true"/>

<one-to-manyclass="Address"/>

</set>

</class>

<class name="Address">

<id name="id"column="addressId">

<generatorclass="native"/>

</id>

</class>

create table Person ( personId bigint not null primary key )

create table Address ( addressId bigint not null primary key, personIdbigint not null )

我们认为对于这种关联关系最好使用连接表。

8.3.使用连接表的单向关联（Unidirectional associationswith join tables）

8.3.1.一对多(oneto many)

基于连接表的单向一对多关联 应该优先被采用。请注意，通过指定unique="true"，我们可以把多样性从多对多改变为一对多。

<class name="Person">

<id name="id"column="personId">

<generatorclass="native"/>

</id>

<setname="addresses" table="PersonAddress">

<keycolumn="personId"/>

<many-to-manycolumn="addressId"

unique="true"

class="Address"/>

</set>

</class>

<class name="Address">

<id name="id"column="addressId">

<generatorclass="native"/>

</id>

</class>

create table Person ( personId bigint not null primary key )

create table PersonAddress ( personId not null, addressId bigint not nullprimary key )

create table Address ( addressId bigint not null primary key )

8.3.2.多对一（manyto one）

基于连接表的单向多对一关联在关联关系可选的情况下应用也很普遍。

<class name="Person">

<id name="id"column="personId">

<generatorclass="native"/>

</id>

<jointable="PersonAddress"

optional="true">

<keycolumn="personId" unique="true"/>

<many-to-onename="address"

column="addressId"

not-null="true"/>

</join>

</class>

<class name="Address">

<id name="id"column="addressId">

<generatorclass="native"/>

</id>

</class>

create table Person ( personId bigint not null primary key )

create table PersonAddress ( personId bigint not null primary key,addressId bigint not null )

create table Address ( addressId bigint not null primary key )

8.3.3.一对一（oneto one）

基于连接表的单向一对一关联非常少见，但也是可行的。

<class name="Person">

<id name="id"column="personId">

<generatorclass="native"/>

</id>

<jointable="PersonAddress"

optional="true">

<keycolumn="personId"

unique="true"/>

<many-to-onename="address"

column="addressId"

not-null="true"

unique="true"/>

</join>

</class>

<class name="Address">

<id name="id"column="addressId">

<generatorclass="native"/>

</id>

</class>

create table Person ( personId bigint not null primary key )

create table PersonAddress ( personId bigint not null primary key,addressId bigint not null unique )

create table Address ( addressId bigint not null primary key )

8.3.4.多对多（manyto many）

最后，还有 单向多对多关联.

<class name="Person">

<id name="id"column="personId">

<generatorclass="native"/>

</id>

<setname="addresses" table="PersonAddress">

<keycolumn="personId"/>

<many-to-manycolumn="addressId"

class="Address"/>

</set>

</class>

<class name="Address">

<id name="id"column="addressId">

<generatorclass="native"/>

</id>

</class>

create table Person ( personId bigint not null primary key )

create table PersonAddress ( personId bigint not null, addressId bigintnot null, primary key (personId, addressId) )

create table Address ( addressId bigint not null primary key )

8.4.双向关联（Bidirectional associations）

8.4.1.一对多（one to many) / 多对一（many to one）

双向多对一关联 是最常见的关联关系。（这也是标准的父/子关联关系。）

<class name="Person">

<id name="id"column="personId">

<generatorclass="native"/>

</id>

<many-to-onename="address"

column="addressId"

not-null="true"/>

</class>

<class name="Address">

<id name="id"column="addressId">

<generatorclass="native"/>

</id>

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

<key column="addressId"/>

<one-to-manyclass="Person"/>

</set>

</class>

create table Person ( personId bigint not null primary key, addressIdbigint not null )

create table Address ( addressId bigint not null primary key )

8.4.2.一对一（one to one）

基于外键关联的双向一对一关联也很常见。

<class name="Person">

<id name="id"column="personId">

<generatorclass="native"/>

</id>

<many-to-onename="address"

column="addressId"

unique="true"

not-null="true"/>

</class>

<class name="Address">

<id name="id"column="addressId">

<generatorclass="native"/>

</id>

<one-to-onename="person"

property-ref="address"/>

</class>

create table Person ( personId bigint not null primary key, addressIdbigint not null unique )

create table Address ( addressId bigint not null primary key )

基于主键关联的一对一关联需要使用特定的id生成器。

<class name="Person">

<id name="id"column="personId">

<generatorclass="native"/>

</id>

<one-to-onename="address"/>

</class>

<class name="Address">

<id name="id"column="personId">

<generatorclass="foreign">

<paramname="property">person</param>

</generator>

</id>

<one-to-onename="person"

constrained="true"/>

</class>

create table Person ( personId bigint not null primary key )

create table Address ( personId bigint not null primary key )

8.5.使用连接表的双向关联（Bidirectional associationswith join tables）

8.5.1.一对多（oneto many） /多对一（ many to one）

基于连接表的双向一对多关联。注意inverse="true"可以出现在关联的任意一端，即collection端或者join端。

<class name="Person">

<id name="id"column="personId">

<generatorclass="native"/>

</id>

<setname="addresses"

table="PersonAddress">

<keycolumn="personId"/>

<many-to-manycolumn="addressId"

unique="true"

class="Address"/>

</set>

</class>

<class name="Address">

<id name="id"column="addressId">

<generatorclass="native"/>

</id>

<jointable="PersonAddress"

inverse="true"

optional="true">

<keycolumn="addressId"/>

<many-to-onename="person"

column="personId"

not-null="true"/>

</join>

</class>

create table Person ( personId bigint not null primary key )

create table PersonAddress ( personId bigint not null, addressId bigintnot null primary key )

create table Address ( addressId bigint not null primary key )

8.5.2.一对一（oneto one）

基于连接表的双向一对一关联极为罕见，但也是可行的。

<class name="Person">

<id name="id"column="personId">

<generatorclass="native"/>

</id>

<jointable="PersonAddress"

optional="true">

<keycolumn="personId"

unique="true"/>

<many-to-onename="address"

column="addressId"

not-null="true"

unique="true"/>

</join>

</class>

<class name="Address">

<id name="id"column="addressId">

<generatorclass="native"/>

</id>

<jointable="PersonAddress"

optional="true"

inverse="true">

<keycolumn="addressId"

unique="true"/>

<many-to-onename="address"

column="personId"

not-null="true"

unique="true"/>

</join>

</class>

create table Person ( personId bigint not null primary key )

create table PersonAddress ( personId bigint not null primary key,addressId bigint not null unique )

create table Address ( addressId bigint not null primary key )

8.5.3.多对多（manyto many）

最后，还有 双向多对多关联.

<class name="Person">

<id name="id"column="personId">

<generatorclass="native"/>

</id>

<setname="addresses">

<keycolumn="personId"/>

<many-to-manycolumn="addressId"

class="Address"/>

</set>

</class>

<class name="Address">

<id name="id"column="addressId">

<generatorclass="native"/>

</id>

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

<key column="addressId"/>

<many-to-manycolumn="personId"

class="Person"/>

</set>

</class>

create table Person ( personId bigint not null primary key )

create table PersonAddress ( personId bigint not null, addressId bigintnot null, primary key (personId, addressId) )

create table Address ( addressId bigint not null primary key )

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第7章集合类(Collections)映射	起始页	第9章组件（Component）映射

来自：http://docs.huihoo.com/hibernate/reference-v3_zh-cn/associations.html