NHibernate双向多对多映射列表/包 [英] NHibernate Bidirectional Many-to-Many Mapping List / Bag

问题描述

我有两个类别，一个是双向多对多映射，其中一个是重要的顺序，另一个则不重要。 ：程序和学生。

一个程序有很多学生，顺序是重要的。

程序A

1. John


2. Sally


3. Seth

程序B

1. Alex


2. Seth


3. Amy


4. 约翰

John
*程式A
*程式B

>

Sally

• 计划A

Seth

• 程式A


• 程式B li>
  

Alex

• 计划B





Amy






• 计划B






所以，它附上ars，我会在程序和学生之间建立一个双向的多对多关系，我可以做这样的事情......

  var john = GetJohn（）; 
 var programCount = john.Programs.Count; // 2 
 
 var programB = GetProgramB（）; 
 var studentCount = programB.Students.Count; // 4 
 var secondStudent = programB.Students [1]; // Seth 
  

我无法弄清楚如何让映射工作。





程序制图





程序制图

 <？xml version =1.0encoding =utf-8？> 
< hibernate-mapping xmlns =urn：nhibernate-mapping-2.2> 
< class name =Programtable =Programslazy =false> 
< id name =IDcolumn =IDtype =Int32unsaved-value =0> 
< generator class =identity/> 
< / id> 
< property name =Titlecolumn =Titletype =Stringlength =200/> 
< list name =Studentstable =ProgramAssignmentslazy =truecascade =all> 
< key column =ProgramID/> 
< index column =SequenceIndextype =Int32/> 
<多对多列=StudentIDclass =Student/> 
< / list> 
< / class> 
< / hibernate-mapping> 
  

学生映射

 < hibernate-mapping xmlns =urn：nhibernate-mapping-2.2> 
< class name =Studenttable =Studentslazy =false> 
< id name =IDcolumn =IDtype =Int32unsaved-value =0> 
< generator class =identity/> 
< / id> 
< property name =Titlecolumn =Titletype =Stringlength =200/> 
< bag name =Programstable =ProgramAssignmentslazy =true> 
< key column =StudentID/> 
 < / bag> 
< / class> 
< / hibernate-mapping> 
  

我的关联表非常简单...

  ProgramID int 
 StudentID int 
 SequenceIndex int 
  

PK是ProgramID + StudentID。





我真的希望从程序端管理这个关联，因为每个程序中的学生顺序是重要。但是，我真的希望能够通过mystudent.Programs访问特定学生的课程。我已经尝试了许多变化的映射，包括在程序列表中设置inverse = true，尝试不同的级联选项等。似乎没有什么可行的。





帮助！感谢！

解决方案

这个问题可能隐藏在< list> mapping支持一个实例的多重赋值（所以可能会有学生 Seth 分配两次，使用不同的 SequenceIndex - 违反PK错误）





但我想给你一个提示。尝试改变方法并引入中间对象。查看NHibernate文档：第24章最佳实践。快速总结：






不要使用外来的关联映射。





真正的多对多关联的好用例很少见。
大部分时间您需要存储在链接表中的附加信息。
在这种情况下，使用两个一对多关联到
中间链接类更好。事实上，我们认为大多数协会
是一对多和多对一的，您在使用任何
其他协会风格时应该小心，并问自己是否真的需要。




我想说，这是（可能）的情况，因为您确实需要订单。 ..




I am struggling with a bi-directional many-to-many mapping where the order is important on one side, but not the other.

I have two classes: Program and Student.

A program has many students and the sequential order is important.

Program A

1. John
2. Sally
3. Seth

Program B

1. Alex
2. Seth
3. Amy
4. John

A student has many programs, but the order is not important here.

John * Program A * Program B

Sally

• Program A

Seth

• Program A
• Program B

Alex

• Program B

Amy

• Program B

So, it appears that I would have a bidirectional many-to-many association between programs and students where I could do things like this...

var john = GetJohn();
var programCount = john.Programs.Count; // 2

var programB = GetProgramB();
var studentCount = programB.Students.Count; // 4
var secondStudent = programB.Students[1]; // Seth

I cannot figure out how to get the mapping to work. I keep getting PK violation errors.

I have tried the following...

PROGRAM MAPPING

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2">
    <class name="Program" table="Programs" lazy="false">
        <id name="ID" column="ID" type="Int32" unsaved-value="0">
            <generator class="identity" />
        </id>
        <property name="Title" column="Title" type="String" length="200" />
        <list name="Students" table="ProgramAssignments" lazy="true" cascade="all">
            <key column="ProgramID" />
            <index column="SequenceIndex" type="Int32" />
            <many-to-many column="StudentID" class="Student" />
        </list>
    </class>
</hibernate-mapping>

STUDENT MAPPING

<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2">
    <class name="Student" table="Students" lazy="false">
        <id name="ID" column="ID" type="Int32" unsaved-value="0">
            <generator class="identity" />
        </id>
        <property name="Title" column="Title" type="String" length="200" />
        <bag name="Programs" table="ProgramAssignments" lazy="true">
            <key column="StudentID" />
            <many-to-many column="ProgramID" class="Program" />
        </bag>
    </class>
</hibernate-mapping>

My association table is very simple...

ProgramID int
StudentID int
SequenceIndex int

The PK is the ProgramID + StudentID.

I really want the association to be managed from the program side because the order of the students in each program is important. But, I really would like to be able to access the programs for a specific student via mystudent.Programs. I have tried many variations of the mapping, including setting inverse=true on the program list, trying different cascade options, etc. Nothing seems to work.

Help! Thanks!

解决方案

The issue could be hidden in the fact, that the <list> mapping supports multiple assignment of one instance (so there could be student Seth assigned twice, with different SequenceIndex - violating the PK error)

But I would like to give you a hint. Try to change the approach and introduce the intermediate object. See the NHibernate documentation: Chapter 24. Best Practices. Quick summary:

Don't use exotic association mappings.

Good usecases for a real many-to-many associations are rare. Most of the time you need additional information stored in the "link table". In this case, it is much better to use two one-to-many associations to an intermediate link class. In fact, we think that most associations are one-to-many and many-to-one, you should be careful when using any other association style and ask yourself if it is really neccessary.

And I would say, that this is (could be) the case, because you really need the Order ...

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