与附加约束

Question

我新的EF和CodeFirst和我有以下（简化）模型导航属性：

public class Comment 
{ 
    public int ID {get; set;} 
    public int SourceType {get; set;} 
    public int SourceID {get; set;} 
    public string Name {get; set;} 
    public string Text {get; set;} 
} 

public class Photo 
{ 
    public int ID {get; set;} 
    public virtual ICollection<Comment> Comments {get; set;} 
} 

public class BlogPost 
{ 
    public int ID {get; set;} 
    public virtual ICollection<Comment> Comments {get; set;} 
} 

在我实际的数据库，我只有茨艾伦三个表。 我的目标是制作一张表格“评论”，用于存储用户发布的照片​​以及博客帖子的评论。 Comment.SourceType字段应区分发布到照片（SourceType == 1）或博客文章（SourceType == 2）的评论，而Comment.SourceID字段告诉我源的ID。

Photo photo = DbContext.Photos.Find(15); //some photo with ID 15 
BlogPost blog = DbContext.BlogPost.Find(15); //some blog post, also with ID 15 

Comment photoComment = new Comment(); 
photoComment.SourceType = 1; //photo 
photoComment.SourceID = photo.ID; 
photoComment.Name = "John"; 
photoComment.Text = "This is a very nice picture!"; 

Comment blogComment = new Comment(); 
blogComment.SourceType = 2; //blog post 
blogComment.SourceID = blog.ID; 
blogComment.Name = "Peter"; 
blogComment.Text = "An interesting blog post!"; 

DbContext.Comments.Add(photoComment); 
DbContext.Comments.Add(blogComment); 
DbContext.SaveChanges(); 

//... 

Photo photoFromBefore = DbContext.Photos.Find(15); 
foreach(Comment comment in photoFromBefore.Comments) 
    Console.Write(comment.Name+"("+comment.SourceType+", "+comment.SourceID+"); "); 
//Output will be: "John(1, 15); Peter(2, 15);" 
//Desired output should be instead just "John(1, 15);" 
//because Peter's comment actually belongs to blog post with 
//the same ID but different "SourceType"-identifier in my database table. 

我希望它能以某种方式清楚我想要达到的目标。基本上，我不希望有多个表photo_comments,blogpost_comments等等，可以在我的网站上发表评论。

我能以某种方式告诉EF仅加载与SourceType==1（对于照片）的评论吗？有什么类型的“约束”或“限制”我可以用来实现这一点？

Answer 1

你应该可以在你的foreach语句中使用LINQ where子句。

foreach(Comment comment in photoFromBefore.Comments.Where(x=>x.SourceType == 2)) 

而且，看代码，应该在foreach上面的线是

IEnumerable<Photo> photoFromBefore = DbContext.Photos.First(15);