MVC :: Many-To-Many Automatic Field Binding?

The short version of this question is "is there a way to gracefully accommodate automatic field binding & database insertion for an object that has a many-to-many field that has been set up in a partial class?" Apologies if it's been asked before.

Example Suppose I have a typical MVC setup with the tables:

Posts {PostID, ...}
Categories {CategoryID, ...}
PostCategories {PostID, CategoryID, ...}

As far as I know, there's no way to do many-to-many relationships in Linq-to-SQL right now so I have to shoehorn it in by adding a partial Post class to the project to add that functionality. Something like:

public partial class Post{ public IEnumerable<Category> Categories{ get { ... } set { ... } }}

So I can now create a "Create" view that automatically populates a "Categories" UI item. It even handles like any other field- fun!

So here's my problem:

How does all of this get exposed to views like "Create" and "Index" and still get to use automatic object model binding? The goal is to a) have the view and automatic model binder treat this as if it were native functionality of the object and b) have the Post object gracefully handle creating PostCategory table entries on submission into the database.