如何使用EF4 Code-First一次创建两个关联实体

Question

So Say I have 2 entities, Post and PostHistory . Whenever I create or edit a post, I want to create an exact copy of it as PostHistory to log all changes made to a post. Say the post entity has the following definition:

public class Post
{
    public int Id { get; set; }
    public string Text { get; set; }
    public virtual ICollection<PostHistory> PostHistory{ get; set; }
}

The problem comes when I create my first Post . For example, if I try this:

Post post = new Post { Text = "Blah" };
context.Posts.Add(post);
PostHistory history = new PostHistory { Text = "Blah"};
context.PostHistory.Add(history);
post.PostHistory.Add(history);

That will fail because since both post and history are new entities, Post.PostHistory is null due to it not having been initialized yet. The only way I can see to do this is to first commit post to the db, then commit history to the database, but I don't see why performing 2 separate inserts should be necessary for this.

If I have a constructor that initializes the ICollection to a List<T> everything works fine, but forcing the implementation to List<T> causes other issues, and almost no code-first tutorials do this forced initiation.

So what's the best way to handle this situation?

Answer 1

You can initialize the list in the client code itself:

Post post = new Post 
{ 
    Text = "Blah" 
    PostHistory = new List() 
    { 
        new PostHistory { Text = "Blah" }
    }
};        
context.Posts.Add(post);
context.SaveChanges();

However, there is nothing wrong with initializing the collection inside the constructor and it is actually recommended because it saves you from having to initialize it in all the client codes.