NHibernate双向一对一映射问题 [英] NHibernate Bi-Directional one-to-one mapping problem

问题描述

当尝试在NHibernate中创建双向一对一映射时，我发现我无法递归获得对象的引用.

While trying to create a Bi-directional one-to-one mapping in NHibernate, I found that, I am unable to have the Reference of the objects recursively.

例如:假设我在Person和Address之间具有一对一的关系.

For example: suppose I have one-to-one relationships between Person and Address.

然后在执行以下代码后，

then after executing the following code,

class Person
{
    ... ...
    public Address Address { get;set; }
}

class Address
{
    ... ...
    public Person Person {get;set;}
}

Repository<Person> rep = new Repository<Person>();
Person p = rep.Get<Person>(1);

我需要从p.Address.Person获得一个非null值. IE. ID为1的同一个人.

I need to have a non-null value from p.Address.Person. I.e. the same person with an ID of 1.

但是该属性返回一个null值.

But the property is returning a null-value.

我应该寻找什么来解决这个问题?

What should I look for to solve the problem?

我的数据库表是这样的:

My database tables are like this:

Address {ID, Desc}
Person {ID, Name, AddressID}

Person.hbm.xml

<?xml version="1.0" encoding="utf-8" ?>

<hibernate-mapping
    xmlns="urn:nhibernate-mapping-2.2"
     default-access="property"
    >
  <class name="NHibernate__BiDirectional__One_To_One.BO.Person, NHibernate__BiDirectional__One_To_One.BO" 
         table="Person">
    <id name="ID">
      <generator class="native" />
    </id>
    <property name="Name"/>

    <many-to-one
        name="Address"
        class="NHibernate__BiDirectional__One_To_One.BO.Address, NHibernate__BiDirectional__One_To_One.BO"
        column="AddressID" 
        cascade="all" 
        unique="true" />

  </class>
</hibernate-mapping>

Address.hbm.xml

<?xml version="1.0" encoding="utf-8" ?>

<hibernate-mapping
  xmlns="urn:nhibernate-mapping-2.2"
   default-access="property"
  >
  <class name="NHibernate__BiDirectional__One_To_One.BO.Address, NHibernate__BiDirectional__One_To_One.BO" 
         table="Address">
    <id name="ID" >
      <generator class="native" />
    </id>
    <property name="Desc"/>      
    <one-to-one
        name="Person"
        class="NHibernate__BiDirectional__One_To_One.BO.Person, NHibernate__BiDirectional__One_To_One.BO"
        />
  </class>
</hibernate-mapping>

我也遇到错误:

could not load an entity: [NHibernate__BiDirectional__One_To_One.BO.Person#1][SQ
L: SELECT person0_.ID as ID0_1_, person0_.Name as Name0_1_, address1_.ID as ID1_
0_, address1_.Desc as Desc1_0_, address1_.AddressID as AddressID1_0_ FROM Person
 person0_ left outer join Address address1_ on person0_.ID=address1_.AddressID W
HERE person0_.ID=?]
Incorrect syntax near the keyword 'Desc'.

推荐答案

一对一关联有两种:

•主键关联

•唯一的外键关联

主键关联不需要额外的表列；如果通过关联将两行相关联，则 两个表行共享相同的主键值.因此，如果您希望通过主键关联来关联两个对象， 您必须确保为它们分配了相同的标识符值！ 对于主键关联，分别将以下映射添加到Employee和Person.

Primary key associations don't need an extra table column; if two rows are related by the association then the two table rows share the same primary key value. So if you want two objects to be related by a primary key association, you must make sure that they are assigned the same identifier value! For a primary key association, add the following mappings to Employee and Person, respectively.

<one-to-one name="Person" class="Person"/>
<one-to-one name="Employee" class="Employee" constrained="true"/>

现在，我们必须确保PERSON和EMPLOYEE表中相关行的主键相等.

Now we must ensure that the primary keys of related rows in the PERSON and EMPLOYEE tables are equal.

我们使用一种特殊的NHibernate标识符生成策略，称为外来策略:

We use a special NHibernate identifier generation strategy called foreign:

<class name="Person" table="PERSON">
<id name="Id" column="PERSON_ID">
<generator class="foreign">
<param name="property">Employee</param>
</generator>
</id>
...
<one-to-one name="Employee"
class="Employee"
constrained="true"/>
</class>

然后为新保存的Person实例分配与Employee实例引用的相同的主键值 具有该人员的Employee属性. 另外，从雇员到人，具有唯一约束的外键可以表示为:

A newly saved instance of Person is then assigned the same primar key value as the Employee instance refered with the Employee property of that Person. Alternatively, a foreign key with a unique constraint, from Employee to Person, may be expressed as:

<many-to-one name="Person" class="Person" column="PERSON_ID" unique="true"/>

并且可以通过在Person映射中添加以下内容来实现双向关联:

<one-to-one name="Employee" class="Employee" property-ref="Person"/>

看看这个

https://forum.hibernate.org/viewtopic.php? p = 2362617& sid = 23c4df33b683409df9b5d844037d6d03

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