Linq deferred join and where

[dlouche]

I have a method that contains the linq query:

var query = from c in db.Categories
                 join pc in db.ProductsToCategories on c.ID equals pc.CategoryID
                 where pc.ProductID == productID
                 orderby c.Name ascending
                 select c;
 

but I would like to make the productID optional (decimal? productID)
So I want to strip the join and the where out of the query and add it back in with deferred calls to .Join and .Where only if required.

However I am having a problem with the where (haven't tried the join yet)

var query = from c in db.Categories
                 join pc in db.ProductsToCategories on c.ID equals pc.CategoryID
                 orderby c.Name ascending
                 select c;
 
query = query.Where(pc => pc.ProductID);
 

It thinks that 'pc' is of type 'c' so it doesn't know what ProductID is.
How do I reference the joined table in a deferred Where call?

Thanks

Dan