Question

With the Entity Framework (EF) I want to load an object from my database, modify it and save it back. However, loading and saving happens in different contexts and I modify it by adding another object to a collection property of the object.

Consider the following code based on the famous blog/posts example from MSDN:

Blog blog;

using (BloggingContext db = new BloggingContext())
{
    blog = db.Blogs.Include("Posts").Single();
}

// No one else knows the `post` object directly.
{
    Post post = new Post {Blog = blog, Title = "Title", Content = "Content"};
    blog.Posts.Add(post);
}

using (BloggingContext db = new BloggingContext())
{
    // No idea what I have to do before saving...
    // Can't do anything with `post` here, since this part will not know this 
    // object directly.

    //db.Blogs.Attach(blog); // throws an InvalidOperationException
    db.SaveChanges();
}

In my database I have 1 Blog object with 100 Posts. As you can see, I want to add a new Post to this Blog. Unfortunately, doing db.Blogs.Attach(blog); before saving, throws an InvalidOperationException saying: “A referential integrity constraint violation occurred: The property values that define the referential constraints are not consistent between principal and dependent objects in the relationship.”

What do I have to do to let the EF update this blog?

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UPDATE:

I think what I was trying to achieve (decoupling the database update of an entity from the modifications and its related child entities) is not possible. Instead, I consider the opposite direction more feasible now: decoupling the update/creation of a child entity from its parent entity. This can be done the following way:

Blog blog;

using (BloggingContext db = new BloggingContext())
{
    blog = db.Blogs.Single();
}

Post post = new Post {BlogId = blog.BlogId, Title = "Title", Content = "..."};

using (BloggingContext db = new BloggingContext())
{
    db.Posts.Add(post);
    db.SaveChanges();
}

Answer 1

You have to attach the entity to the context and then change tracking should kick in and save changes will do the rest.

For reference: MSDN Attach Entities to Context

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Or try adding it explicitly and set the relationship needed information directly and not through the navigation property like so:

Blog blog;

using (BloggingContext db = new BloggingContext())
{
    blog = db.Blogs.Include("Posts").Single();

    Post post = new Post {Blog = blog, Title = "Title", Content = "Content"};
    post.blogId = blog.Id;    

    db.Posts.Add(post);
    db.SaveChanges();
}