Hibernate一对一关联实用介绍

#0
书和文档上写的都不是特清楚的。自己记下来。

#1 Using a PK association

#1.1 POJO with XDolclet annotation
public class Customer {
 /**
  * @return Returns the shoppingCart.
  * @hibernate.many-to-one cascade="delete" column="shopping_cart_id"
  *  unique="true" foreign-key="FK_SHOPPING_CART__CUSTOMER"
  */
 public ShoppingCart getShoppingCart() {
  return shoppingCart;
 }
}

public class ShoppingCart {
 /**
  * @return Returns the customer.
  * @hibernate.one-to-one property-ref="shoppingCart"
  */
 public Customer getCustomer() {
  return customer;
 }
}

property-ref="shoppingCart" 告诉Hibernate ShoppingCart#customer和Customer#shoppingCart是反向的关系。所以Hibernate知道如何从ShoppingCart#getCustomer中检索到相应的customer对象。取出某个Customer对象时，Hibernate会生成DEBUG SQL:324 - 3中的SQL语句。

#1.2 HBM
Customer.hbm.xml
<many-to-one
    name="shoppingCart"
    class="ShoppingCart"
    cascade="delete"
    outer-join="auto"
    foreign-key="FK_SHOPPING_CART__CUSTOMER"
    column="shopping_cart_id"
/>

ShoppingCart.hbm.xml
<one-to-one
    name="customer"
    class="Customer"
    cascade="none"
    outer-join="auto"
    constrained="false"
    property-ref="shoppingCart"
/>

#1.3 SCHEMA SQL
create table CUSTOMER (
    ID bigint generated by default as identity (start with 1),
    SHOPPING_CART_ID bigint,
    primary key (ID)
)

create table SHOPPING_CART (
    ID bigint generated by default as identity (start with 1)
    primary key (ID)
)

alter table CUSTOMER
    add constraint FK_SHOPPING_CART__CUSTOMER
    foreign key (SHOPPING_CART_ID)
    references SHOPPING_CART

#1.4 Query SQL
DEBUG SQL:324 - 1
select customer0_.ID as ID, customer0_.SHOPPING_CART_ID as SHOPPING2_3_, customer0_.USERNAME as USERNAME3_, customer0_.PWD as PWD3_
from CUSTOMER customer0_
where customer0_.USERNAME=? and customer0_.PWD=?

DEBUG SQL:324 - 2
select shoppingca0_.ID as ID0_, shoppingca0_.TOTAL as TOTAL8_0_
from SHOPPING_CART shoppingca0_
where shoppingca0_.ID=?

DEBUG SQL:324 - 3
select customer0_.ID as ID1_, customer0_.SHOPPING_CART_ID as SHOPPING2_3_1_, customer0_.USERNAME as USERNAME3_1_, customer0_.PWD as PWD3_1_, shoppingca1_.ID as ID0_, shoppingca1_.TOTAL as TOTAL8_0_
from
 CUSTOMER customer0_
 left outer join
 SHOPPING_CART shoppingca1_
 on customer0_.SHOPPING_CART_ID=shoppingca1_.ID
where customer0_.SHOPPING_CART_ID=?

#2 Using a FK association

#2.1 POJO with XDolclet annotation
public class Customer {
 /**
  * @return Returns the shoppingCart.
  * @hibernate.one-to-one cascade="delete"
  */
 public ShoppingCart getShoppingCart() {
  return shoppingCart;
 }
}

public class ShoppingCart {
 /**
  * @return Returns the id.
  * @hibernate.id generator-class="foreign"
  * @hibernate.generator-param name="property" value="customer"
  */
 public Long getId() {
  return id();
 }

 /**
  * @return Returns the customer.
  * @hibernate.one-to-one constrained="true" foreign-key="FK_CUSTOMER__SHOPPING_CART"
  */
 public Customer getCustomer() {
  return customer;
 }
}

constrained="true" 告诉Hibernate ShoppingCart的PK还应该是一个FK，这个FK引用Customer的PK。还需要多做一点工作，声明ShoppingCart的PK生成策略是foreign，对应ShoppingCart#customer。这和上面一句话不是一个意思嘛，FT~~

#2.2 HBM
Customer.hbm.xml
<one-to-one
    name="shoppingCart"
    class="ShoppingCart"
    cascade="delete"
    outer-join="auto"
    constrained="false"
/>

ShoppingCart.hbm.xml
<id
    name="id"
    column="id"
    type="java.lang.Long"
>
    <generator class="foreign">
 <param name="property">customer</param>
    </generator>
</id>

<one-to-one
    name="customer"
    class="Customer"
    cascade="none"
    outer-join="auto"
    constrained="true"
/>

#2.3 SCHEMA SQL
create table CUSTOMER (
    ID bigint generated by default as identity (start with 1),
    primary key (ID)
)

create table SHOPPING_CART (
    ID bigint not null,
    TOTAL integer,
    primary key (ID)
)

alter table SHOPPING_CART
    add constraint FK_CUSTOMER__SHOPPING_CART
    foreign key (ID)
    references CUSTOMER

#2.4 Query SQL
DEBUG SQL:324 -
select customer0_.ID as ID, customer0_.USERNAME as USERNAME3_, customer0_.PWD as PWD3_
from CUSTOMER customer0_
where customer0_.USERNAME=? and customer0_.PWD=?

DEBUG SQL:324 -
select shoppingca0_.ID as ID0_, shoppingca0_.TOTAL as TOTAL8_0_
from SHOPPING_CART shoppingca0_
where shoppingca0_.ID=?

这个“真正”的one-to-one的好处是少条关联SQL语句，看到了吗？