Order entitydatasource records

I'm trying to order entitydatasource records like this: orderby="Product_Num" but it says:

开发者_开发问答'Product_Num' is not a member of 'Transient.collection[productmodel.Products(Nullable=True,DefaultValue=)]'. To extract a property of a collection element, use a subquery to iterate over the collection.

So, it means that It cannot order collection of Products. It needs some kind of subquery, but I'm not sure what exactly does it need.