NHibernate 3.2 CodeFirst Many to Many

一直沒有在項目里用過NHibernate3.2的CodeFirst 這次因為項目需要用到Oracle數據庫而我對EF對Oracle支持心里犯嘀咕所以為保險起見我用NHibernate,以前都是寫就映射文件的，現在3.2里已經集成了Maping Bycode工具可以說是NHibernate對CodeFirst支持得很不錯了！我也不想采用Xml文件了（至於原因大家都懂得的！）

明白了大多數的NHibernate映射參數采用CodeFist也應該不是什么難事！

所以今天就試了一把，以前也試過但只粗略的試了一下！出現了以下問題，（結果谷爹 度娘 一起用也沒查到個所以然！只是找了一堆EFCodeFirst的東西，NHibernate的幾乎很少）小弟真不知道是怎么回事兒，所以特向園子里的大俠請教!

廢話不多說了，先上代碼：

 public class Student
    {
        public virtual int StudentID { set; get; }
        public virtual string StudentName { set; get; }
        public virtual IList<CurrClass> CurrClasses { set; get; }
    }

    public class CurrClass
    {
        public virtual int ClassID { set; get; }
        public virtual int ClassName { set; get; }
      
        public virtual IList<Student> Students { set; get; }
    }

    public class CurrClassMap : ClassMapping<CurrClass>
    {
        public CurrClassMap()
        {
            this.Id(p => p.ClassID, map => {
                map.Generator(Generators.Native);
            });

            this.Property(p => p.ClassName);
           
            this.Bag(p => p.Students, map => {
                map.Table("Student_Class");
            });
        }
    }

    public class StudentMap : ClassMapping<Student>
    {
        public StudentMap()
        {
            this.Id(p => p.StudentID, map =>
            {
                map.Generator(Generators.Native);
            });
            this.Property(p => p.StudentName, map =>
            {
                map.Length(500);
            });
            this.Bag(p => p.CurrClasses, map => {
                map.Table("Student_Class");
            });
        }
    }

 用過的朋友一看就知道是個多對多的關系！看似沒什么問題,那么我們來看下生成映射文件是不是夠標准:

<?xml version="1.0" encoding="utf-8"?>
<hibernate-mapping xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" namespace="DALTestCase.domain" assembly="DALTestCase" xmlns="urn:nhibernate-mapping-2.2">
  <class name="CurrClass">
    <id name="ClassID" type="Int32">
      <generator class="native" />
    </id>
    <property name="ClassName" />
    <bag name="Students" table="Student_Class">
      <key column="currclass_key" />
      <many-to-many class="Student" />
    </bag>
  </class>
  <class name="Student">
    <id name="StudentID" type="Int32">
      <generator class="native" />
    </id>
    <property name="StudentName" length="500" />
    <bag name="CurrClasses" table="Student_Class">
      <key column="student_key" />
      <many-to-many class="CurrClass" />
    </bag>
  </class>
</hibernate-mapping>

很不錯！和手寫的沒兩樣！

再來看看我們生成的數據庫腳本（我這里是用的Sql2005）：

if exists (select 1 from sys.objects where object_id = OBJECT_ID(N'[FKCF48CF07D9A53447]') AND parent_object_id = OBJECT_ID('Student_Class'))
alter table Student_Class  drop constraint FKCF48CF07D9A53447


    if exists (select 1 from sys.objects where object_id = OBJECT_ID(N'[FKCF48CF07401A93]') AND parent_object_id = OBJECT_ID('Student_Class'))
alter table Student_Class  drop constraint FKCF48CF07401A93


    if exists (select 1 from sys.objects where object_id = OBJECT_ID(N'[FKCF48CF077158C5E0]') AND parent_object_id = OBJECT_ID('Student_Class'))
alter table Student_Class  drop constraint FKCF48CF077158C5E0


    if exists (select * from dbo.sysobjects where id = object_id(N'CurrClass') and OBJECTPROPERTY(id, N'IsUserTable') = 1) drop table CurrClass

    if exists (select * from dbo.sysobjects where id = object_id(N'Student_Class') and OBJECTPROPERTY(id, N'IsUserTable') = 1) drop table Student_Class

    if exists (select * from dbo.sysobjects where id = object_id(N'Student') and OBJECTPROPERTY(id, N'IsUserTable') = 1) drop table Student

    create table CurrClass (
        ClassID INT IDENTITY NOT NULL,
       ClassName INT null,
       primary key (ClassID)
    )

    create table Student_Class (
        currclass_key INT not null,
       elt INT not null,///這里就是我說的莫名奇妙的多出來了一個字段
       student_key INT not null
    )

    create table Student (
        StudentID INT IDENTITY NOT NULL,
       StudentName NVARCHAR(500) null,
       primary key (StudentID)
    )

    alter table Student_Class 
        add constraint FKCF48CF07D9A53447 
        foreign key (elt) 
        references Student

    alter table Student_Class 
        add constraint FKCF48CF07401A93 
        foreign key (currclass_key) 
        references CurrClass

    alter table Student_Class 
        add constraint FKCF48CF077158C5E0 
        foreign key (student_key) 
        references Student

正在我看的時侯突然發現，第三個表里竟然多了一個elt的字段，到底是怎么回事兒呢！？現在我也沒鬧明白！怎么會憑空多個字段出來！？

結果表結構就變成了下面的這個樣子：

 

至於生成的涉及Student_Class的這個表生成的SQL語句都會存在這個elt的字段。如果操作Student_Class表生成的SQL語句會出現不能執行的錯誤！

為什么會出現elt的字段？為什么用Xml就不會出現elt的字段？這個elt有什么用處！？

希望大俠們可以幫我解惑！！！