实体框架在不同集合中多次引用同一对象

Question

I was just playing around with this Code First example for Entity Framework and I encountered a problem which I don't understand. Assuming that the POCO objects and the database context are the same as in the example and that I have the following code in my application's entry point:

var blog = new Blog { Name = "My Blog" };
var post = new Post {Title = "A Random Post", Blog = blog};
blog.Posts = new List<Post> {post};
db.Blogs.Add(blog);

var blog2 = new Blog {Name = "Another Blog", Posts = new List<Post> {post}};
db.Blogs.Add(blog2);

db.SaveChanges();

// Display all Blogs from the database
var query = from b in db.Blogs
            orderby b.Name
            select b;

Console.WriteLine("All blogs in the database:");
foreach (var item in query)
{
    Console.WriteLine(item.Name);
    foreach (var p in item.Posts)
    {
        Console.WriteLine(p.Title);
    }
}

Why does the post, which is referenced in both blogs, appear only in the second blog? Do I have to call SaveChanges every time I add something to the database?

Answer 1

A post can only belong to one blog. By adding it to the list of posts of the second blog, the reference to the first blog gets overwritten.

Answer 2

It appears that your Blog and Post entities have a many-to-many relationship. Have you configured this relationship in the DbContext?

Example here: EF Code First working with many-to-many relationships

If many-to-many is not your intent, then see Kristof's answer.