MVC 3中的MySQL：不支持指定的方法

Question

in my MVC 3 project I'm usint the MySQL connector. When I try to invoke that method, I get the Specified method is not supported error. Why ?

    public IList<Order> GetOrders()
    {
        return (from x in db.Order 
                where (x.Address.FirstOrDefault() != null) 
                orderby x.Created //it's a DateTime
                descending select x).ToList(); 
    }

edit

I see there's a problem with the x.Address.FirstOrDefault() != null

edit2

That code also works

return (from x in db.Order
             from y in db.Address
             where (y.OrderID == x.OrderID)
        orderby x.Created descending
        select x).ToList();

Answer 1

With some ORM tools, not all of the LINQ methods are supported out of the box. I know I have had that exact message using NHibernate in the past. In order to achieve this you'll have to rewrite your logic to use more traditional methods. Try this:

return (from x in db.Order 
            where x.Address.Count() > 0 
            orderby x.Created //it's a DateTime
            descending select x).ToList();

Or if for some reason you don't like the above, your other alternative is to first bring the collection in memory by resolving it, then perform your logic (although bear in mind this will be slower as it's bringing more data back from the database):

return (from x in db.Order
            orderby x.Created //it's a DateTime
            descending select x)
       .ToList() //resolve the query, now work with it in memory from here
       .Where(x => x.Address.FirstOrDefault() != null)
       .ToList();