why inverse must be setted in hibernate bidirectional association of one-to-many and many-to-many

If you’ve used CMP 2.0/2.1, you’re familiar with the concept of a managed association (or managed relationship). CMP associations are called container-managed relationships (CMRs) for a reason. Associations in CMP are inherently bidirectional:A change made to one side of an association is instantly reflected at the other side. For example, if we call bid.setItem(item), the container automatically calls item.getBids().add(item).

Transparent POJO-oriented persistence implementations such as Hibernate do
not implement managed associations. Contrary to CMR, Hibernate associations are
all inherently unidirectional. As far as Hibernate is concerned, the association from Bid to Item is a different association than the association from Item to Bid.
文中Item和Bid是one-to-many关系

这段话告诉我们hibernate不会向CMP那样由容器管理对象间的关系(assotiation),hibernate中的对象先天是unidirectional（单向的）;hibernate中，对象有两种关系:unidirection、bidirection,对象的双向关系通过inverse指明，inverse属性告诉Hibernate “inverse端”维护这种关系：one-to-many的many端或many-to-many的link table

Making the association bidirectional

So far so good. But we also need to be able to easily fetch all the bids for a particular item. We need a bidirectional association here, so we have to add scaffolding code to the Item class:
public class Item {...
private Set bids = new HashSet();
public void setBids(Set bids) {
this.bids = bids;
}
public Set getBids() {
return bids;}
public void addBid(Bid bid) {
bid.setItem(this);
bids.add(bid);
}
...
}
You can think of the code in addBid() (a convenience method) as implementing
a managed association in the object model.
A basic mapping for this one-to-many association would look like this:
<class
name="Item"
table="ITEM">
...
<set name="bids">
<key column="ITEM_ID"/>
<one-to-many class="Bid"/>
</set>
</class>

The column mapping defined by the <key> element is a foreign key column of the
associated BID table. Notice that we specify the same foreign key column in this collection mapping that we specified in the mapping for the many-to-one association.
The table structure for this association mapping is shown in figure 3.11.
Now we have two different unidirectional associations mapped to the same foreign key, which poses a problem. At runtime, there are two different in-memory representations of the same foreign key value: the item property of Bid and an element of the bids collection held by an Item. Suppose our application modifies the association by, for example, adding a bid to an item in this fragment of the addBid() method:
bid.setItem(item);//从而bid的ItemId字段有值，为item.id；否则,hibernate更新为null
bids.add(bid);
This code is fine, but in this situation, Hibernate detects two different changes to the in-memory persistent instances. From the point of view of the database, just one value must be updated to reflect these changes: the ITEM_ID column of the BID table. Hibernate doesn’t transparently detect the fact that the two changes refer to the same database column, since at this point we’ve done nothing to indicate that this is a bidirectional association.

We need one more thing in our association mapping to tell Hibernate to treat
this as a bidirectional association: The inverse attribute tells Hibernate that the collection is a mirror image of the many-to-one association on the other side:
<class
name="Item"
table="ITEM">
...
<set
name="bids"
inverse="true">
<key column="ITEM_ID"/>
<one-to-many class="Bid"/>
</set>
</class>

事实上,在one-to-many关系中，不设置invers="true"，hibernate也会产生预期结果，只是会多执行几条无用的update语句（这些update看起来有点莫名其妙），一般情况下不会影响执行结果；

about many-to-many
When you map a bidirectional many-to-many association, you must declare one
end of the association using inverse="true" to define which side’s state is used to update the link table.same as one-to-many

why inverse must be setted in hibernate bidirectional association of one-to-many and many-to-many相关推荐

hibernate官方新手教程 (转载)
hibernate官方新手教程第一部分 - 第一个Hibernate程序首先我们将创建一个简单的控制台(console-based)Hibernate程序.我们使用内置数据库(in-memory d ...
HIBERNATE - 符合Java习惯的关系数据库持久化 Hibernate参考文档
Hibernate参考文档 3.1.2 目录前言 1. 翻译说明 2. 版权声明 1.Hibernate入门 1.1.前言 1.2.第一部分-第一个Hibernate应用程序 1.2.1.第一个cl ...
Hibernate中文参考文档（JFIS)
HIBERNATE - 符合Java习惯的关系数据库持久化下一页 HIBERNATE - 符合Java习惯的关系数据库持久化 Hibernate参考文档 3.0.4 目录前言 1. 翻译 ...
Hibernate框架--学习笔记（中）：一对多配置、多对多配置
一.一对多: 一个客户可以有多个联系人,一个联系人只能属于一个客户. 1.一对多映射配置: (1)创建实体类 //客户类:一个客户可以有多个联系人,一个联系人只能属于一个客户 public class ...
inverse和Cascade详解
Hibernate中的inverse在表关系映射中经常应用, inverse的值有两种,"true"和"false".inverse="false&q ...
Hibernate（三） - hibernate 表操作-多对多配置
Hibernate 的一对多关联映射之前在学习 Hibernate 的时候,其实都是单表的操作.在实际的开发当中,比如做一个商城,就需要好多张数据库表,表与表之间是有关系的.之前些做一些关联查询或者 ...
Hibernate中的级联一对多关系
两个pojo,Baby 和Baby File 对应数据库中表baby和babyfile.两个表有外键关联,且babyfile的外键不能为空配置了个单向一对多,级联关系为save-update /** ...
Hibernate 常见异常
Hibernate 常见异常 net.sf.hibernate.MappingException 当出现net.sf.hibernate.MappingException: Error ...
表删除时 Cannot delete or update a parent row: a foreign key constraint fails 异常处理
有两张表,结构如下: t_item: t_bid: id int id int name varchar name varchar item_id int 其中表t_item的主键id是表t_bid的 ...

why inverse must be setted in hibernate bidirectional association of one-to-many and many-to-many

why inverse must be setted in hibernate bidirectional association of one-to-many and many-to-many相关推荐

最新文章

热门文章