用自定义的联接表创建多对多关系是不可能的。在多对多关系中，EF内部管理和隐藏联接表。这是一个在模型中没有Entity类的表。要使用具有其他属性的联接表，您实际上必须创建两个一对多关系。它可能看起来像这样：
public class Member
{
public int MemberID { get; set; }
public string FirstName { get; set; }
public string LastName { get; set; }
public virtual ICollection<MemberComment> MemberComments { get; set; }
}
public class Comment
{
public int CommentID { get; set; }
public string Message { get; set; }
public virtual ICollection<MemberComment> MemberComments { get; set; }
}
public class MemberComment
{
[Key, Column(Order = 0)]
public int MemberID { get; set; }
[Key, Column(Order = 1)]
public int CommentID { get; set; }
public virtual Member Member { get; set; }
public virtual Comment Comment { get; set; }
public int Something { get; set; }
public string SomethingElse { get; set; }
}
例如，如果您现在想查找带有LastName=“ Smith” 的成员的所有注释，则可以编写如下查询：
var commentsOfMembers = context.Members
.Where(m => m.LastName == "Smith")
.SelectMany(m => m.MemberComments.Select(mc => mc.Comment))
.ToList();
... 要么 ...
var commentsOfMembers = context.MemberComments
.Where(mc => mc.Member.LastName == "Smith")
.Select(mc => mc.Comment)
.ToList();
或创建一个名称为“ Smith”（假设有多个）的成员及其评论的列表，您可以使用投影：
var membersWithComments = context.Members
.Where(m => m.LastName == "Smith")
.Select(m => new
{
Member = m,
Comments = m.MemberComments.Select(mc => mc.Comment)
})
.ToList();
如果要查找MemberId= 1 的成员的所有注释：
var commentsOfMember = context.MemberComments
.Where(mc => mc.MemberId == 1)
.Select(mc => mc.Comment)
.ToList();
现在，您还可以按联接表中的属性进行过滤（在多对多关系中这是不可能的），例如：过滤成员1中属性为99的所有注释Something：
var filteredCommentsOfMember = context.MemberComments
.Where(mc => mc.MemberId == 1 && mc.Something == 99)
.Select(mc => mc.Comment)
.ToList();
由于延迟加载，事情可能会变得更容易。如果您已加载Member，则无需显式查询即可获得注释：
var commentsOfMember = member.MemberComments.Select(mc => mc.Comment);
我想延迟加载会自动在后台获取评论。
编辑
仅出于娱乐目的，在此模型中还提供了一些示例，说明如何添加实体和关系以及如何删除它们：
1）创建一个成员和该成员的两个评论：
var member1 = new Member { FirstName = "Pete" };
var comment1 = new Comment { Message = "Good morning!" };
var comment2 = new Comment { Message = "Good evening!" };
var memberComment1 = new MemberComment { Member = member1, Comment = comment1,
Something = 101 };
var memberComment2 = new MemberComment { Member = member1, Comment = comment2,
Something = 102 };
context.MemberComments.Add(memberComment1); // will also add member1 and comment1
context.MemberComments.Add(memberComment2); // will also add comment2
context.SaveChanges();
2）添加member1的第三条评论：
var member1 = context.Members.Where(m => m.FirstName == "Pete")
.SingleOrDefault();
if (member1 != null)
{
var comment3 = new Comment { Message = "Good night!" };
var memberComment3 = new MemberComment { Member = member1,
Comment = comment3,
Something = 103 };
context.MemberComments.Add(memberComment3); // will also add comment3
context.SaveChanges();
}
3）创建新成员并将其与现有评论2关联：
var comment2 = context.Comments.Where(c => c.Message == "Good evening!")
.SingleOrDefault();
if (comment2 != null)
{
var member2 = new Member { FirstName = "Paul" };
var memberComment4 = new MemberComment { Member = member2,
Comment = comment2,
Something = 201 };
context.MemberComments.Add(memberComment4);
context.SaveChanges();
}
4）在现有的member2和comment3之间创建关系：
var member2 = context.Members.Where(m => m.FirstName == "Paul")
.SingleOrDefault();
var comment3 = context.Comments.Where(c => c.Message == "Good night!")
.SingleOrDefault();
if (member2 != null && comment3 != null)
{
var memberComment5 = new MemberComment { Member = member2,
Comment = comment3,
Something = 202 };
context.MemberComments.Add(memberComment5);
context.SaveChanges();
}
5）再次删除此关系：
var memberComment5 = context.MemberComments
.Where(mc => mc.Member.FirstName == "Paul"
&& mc.Comment.Message == "Good night!")
.SingleOrDefault();
if (memberComment5 != null)
{
context.MemberComments.Remove(memberComment5);
context.SaveChanges();
}
6）删除member1及其与注释的所有关系：
var member1 = context.Members.Where(m => m.FirstName == "Pete")
.SingleOrDefault();
if (member1 != null)
{
context.Members.Remove(member1);
context.SaveChanges();
}
该删除的关系MemberComments也因为之间的一个一对多的关系Member和MemberComments与之间Comment并MemberComments是建立与惯例级联删除。这是因为该情况MemberId，并CommentId在MemberComment被检测为外键的属性Member和Comment导航性能，因为FK属性的类型非空的int，需要的关系，并最终导致级联，删除，设置。我认为在这种模型中是有道理的。
