與附加約束

Question

我新的EF和CodeFirst和我有以下（簡化）模型導航屬性：

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;} 
} 

在我實際的數據庫，我只有茨艾倫三個表。 我的目標是製作一張表格「評論」，用於存儲用戶發佈的照片​​以及博客帖子的評論。 Comment.SourceType字段應區分發布到照片（SourceType == 1）或博客文章（SourceType == 2）的評論，而Comment.SourceID字段告訴我源的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. 

我希望它能以某種方式清楚我想要達到的目標。基本上，我不希望有多個表photo_comments,blogpost_comments等等，可以在我的網站上發表評論。

我能以某種方式告訴EF僅加載與SourceType==1（對於照片）的評論嗎？有什麼類型的「約束」或「限制」我可以用來實現這一點？

Answer 1

你應該可以在你的foreach語句中使用LINQ where子句。

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

而且，看代碼，應該在foreach上面的線是

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