EF4 - custom ObjectContext and inheritance question - entity-framework

Question

Spinning further on the previous question I had. Let's say I inherit BlogEntry and Comment from Post. I now want to customize them a bit. A comment to a blog post does not need a title but a comment needs a user reference so I move these two fields out from Post and into Comment and Blog entry like this:
public abstract class Post
{
public virtual int Id { get; set; }
public virtual string Text { get; set; }
public virtual DateTime CreatedAt { get; set; }
}
public class BlogEntry : Post
{
public virtual string Header { get; set; }
public virtual Blog Blog { get; set; }
public virtual IEnumerable<Comment> Comments { get; set; }
}
public class Comment : Post
{
public virtual string Header { get; set; }
public virtual int UserId { get; set; }
public virtual BlogEntry BlogEntry { get; set; }
}
Now I create my custom object context:
public class EntityContext : System.Data.Objects.ObjectContext
{
public EntityContext() : base("name=Entities", "Entities")
{
this.Blogs = CreateObjectSet<Blog>();
this.Posts = CreateObjectSet<Post>();
this.Entries = CreateObjectSet<BlogEntry>();
this.Comments = CreateObjectSet<Comment>();
}
public ObjectSet<Blog> Blogs { get; set; }
public ObjectSet<Post> Posts { get; set; }
public ObjectSet<BlogEntry> Entries { get; set; }
public ObjectSet<Comment> Comments { get; set; }
}
This gives me the following actually quite descriptive error message:
Test method threw exception:
System.ArgumentException: There are
no EntitySets defined for the
specified entity type 'BlogEntry'. If
'BlogEntry' is a derived type, use the
base type instead. For example, you
would see this error if you called
CreateObjectSet()
and DiscontinuedProduct is a known
entity type but is not directly mapped
to an EntitySet. The
DiscontinuedProduct type may be a
derived type where the parent type is
mapped to the EntitySet or the
DiscontinuedProduct type might not be
mapped to an EntitySet at all.
Parameter name: TEntity
Now I am not a master of inheritance and stuff but the way I see this would be to add a set of Entries and Comments as ObjectSet< Post> that would solve my problems?

Answer 1

If you create an ObjectSet for a base type (i.e. Post) you can't create one for derived types too, because you can retrieve instances of all types in the inheritance hierarchy from that one ObjectSet.
i.e. ctx.Posts.OfType<BlogEntry>() would return BlogEntry(s).
So the answer is to simply remove the other two sets.
If yo need to do an add for example you can do this:
ctx.Posts.AddObject(new BlogEntry {....});
no problem at all.
To help you write queries more easily you could probably add a couple of properties to your ObjectContext that look like this:
public ObjectQuery<BlogEntity> Blogs{
get{
return ctx.Posts.OfType<BlogEntry>() as ObjectQuery<BlogEntry>;
}
}
and the same for comments.
Hope this helps
Alex