Entity Framework 4.0 POCO and Many-to-Many problem

Question

I have created POCO domain objects that map to the entities in the entity domain model. Everything was working fine until now when I have to work with the many-to-many relationship.

Let's say I have three tables.
- Blog
- BlogTag
- Tag

You can see that Blogs and Tags are many-to-many with a bridge table, BlogTag that contains a foreign key to both tables.

I also have corresponding domain models:
- Blogs
- BlogsTags
- Tags

Now, I select a list of blogs and I'm trying to access a particular tag from a blog.

myBlog.BlogsTags[0].Tag

BlogTags[0].TagForeignKey is filled in, but BlogTags[0].Tag is null !!

I also have LazyLoading turned on.

What could I be doing wrong?

Thanks.

Okay. Here's some source code.

my context class

public class MyContext : ObjectContext
    {
        public MyContext() : base(Utility.GetConnectionString(...), "containerName")
        {
            Blogs = CreateObjectSet<Blog>();
            BlogsTags = CreateObjectSet<BlogTag>();
            Tags = CreateObjectSet<Tags>();

            base.ContextOptions.LazyLoadingEnabled = true;
        }

        public ObjectSet<Blog> Blogs { get; private set; }
        public ObjectSet<BlogTag> BlogsTags { get; private set; }
        public ObjectSet<Tags> Tags { get; private set; }
    }

and my poco classes just have a list of related objects with the virtual keyword.

Answer 1

BlogTag shouldn't be an entity at all : it is only a relationship, it doesn't contain any actual data. If the relationship is properly modeled in your database using foreign keys, the Entity Model designer should realize that and eliminate BlogTag from the conceptual model...

---

EDIT:

I'm not sure why lazy loading doesn't work here, but you could always use Include to load the Tag explicitly :

var myBlog = context.Blogs.Include("BlogTags.Tag").First(b => b.Id = blogId);
var tag = myBlog.BlogsTags[0].Tag;

Answer 2

Be sure that the relationship properties that you are accessing in the Entity are defined as "virtual" otherwise they will not be traversed automatically.

I had the same problem...eg. of my user entity:

public class User : EntityBase
    {
        public int UserID { get; set; }
        public string Username { get; set; }
        public string Email { get; set; }
        public virtual List<Role> Roles { get; set; } //VIRTUAL here is KEY!
}

I assume your's should be:

 public class Blog
        {

     public string Owner { get; set; }
     public string BlogText { get; set; }
     public virtual List<BlogTag> BlogTags { get; set; }  //VIRTUAL here is KEY!

    }