Many To Many Mappings in Entity Framework

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原文链接：http://www.cnblogs.com/extensivewang/archive/2010/09/23/1833428.html

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#数据库

本文介绍如何使用Entity Framework的CodeFirst方法来定义实体类及其配置，实现数据库与实体模型的映射，包括具体代码示例。

NET 3.5 SP1中的Entity Framework着重于“数据库优先（Database First）”，即从数据库中逆工程生成实体数据模型。而Visual Studio 2010/.NET 4.0中的Entity Framework 4.0引进了“模型优先（Model First）”，即允许你先生成实体数据模型，然后生成映射到该模型的数据库。而Entity Framework Feature CTP1版则更进一步，推出了“只用代码（Code Only）”的预览版，不再需要EDMX文件

Visual Studio 2010 Beta 1
Entity Framework Feature CTP1 (同时参考Entity Framework Feature CTP 1一文，如果你对该CTP中其他特性，特别是POCO模板等，感兴趣的话)
本地的SQL Server 2008 Express或者SQL Server

Code Only全程示范many to many Relationships
1、Database

2、Map 到Entity

(1)course Entity

public class Course : IEntity
{
    private readonly ICollection<CourseCategory> _courseCategorys;

    public Course()
    {
      _courseCategorys = new List<CourseCategory>();
    }

    public virtual ICollection<CourseCategory> CourseCategorys
    {
      [DebuggerStepThrough]
      get { return _courseCategorys; }
    }

    public virtual DateTime ModifiedDate { get; set; }

    public virtual DateTime CreatedDate { get; set; }

    public virtual string Title { get; set; }

}

(2)CourseCategory

  public class CourseCategory : IEntity
{
    private readonly ICollection<Course> _courses;

    public CourseCategory()
    {
      _courses = new List<Course>();
    }

    public virtual string Title { get; set; }

    public virtual string Description { get; set; }

    public virtual DateTime ModifiedDate { get; set; }

    public virtual DateTime CreatedDate { get; set; }
}

注：这里的CourseInCategory只是存放2者之间的关系，那么不需要map到Entity中

3、Configuration

(1）Course

public class CourseConfiguration : EntityConfiguration<Course>
{
    public CourseConfiguration()
    {
      HasKey(c => c.Id);

      Property(c => c.Id).IsIdentity();

      Property(c => c.ModifiedDate);

      Property(c => c.CreatedDate);

      Property(c => c.Title).IsUnicode().IsRequired().HasMaxLength(50);

      MapSingleType(c => new
      {
        CourseID = c.Id,
        c.CreatedDate,
        c.Description,
        c.ModifiedDate,
        c.Title,
      }).ToTable("Course");
    }
}

(2)CourseCategory

  public class CourseCategoryConfiguration : EntityConfiguration<CourseCategory>
{
    public CourseCategoryConfiguration()
    {
      HasKey(cc => cc.Id);

      Property(cc => cc.Id).IsIdentity();

      Property(cc => cc.Title).HasMaxLength(50).IsUnicode().IsRequired();

      Property(cc => cc.Description).HasMaxLength(250).IsUnicode().IsRequired();

      Property(cc => cc.CreatedDate);

      Property(cc => cc.EducationId);

      Property(cc => cc.ModifiedDate);

      Relationship(cc => cc.Courses).FromProperty(c => c.CourseCategorys).Map("CourseInCategory", (cc, c) => new { CourseID = c.Id, CategoryID = cc.Id });

      MapSingleType(cc => new
      {
        CategoryID = cc.Id,
        cc.Title,
        cc.Description,
        cc.CreatedDate,
        cc.EducationId,
        cc.ModifiedDate,
        cc.ParentCategoryId,
      }).ToTable("CourseCategory");
    }
}