Question

I am trying to break up linq to sql queries to make them a bit more readable.

Say I want to return all orders for product which in the previous year had more than 100 orders. I have this query:

from o in _context.Orders
where (from o1 in _context.Orders 
       where o1.Year == o.Year - 1 && o1.Product == o.Product
       select o1).Count() > 100
select o;

What I’d like to be able to do is to put the nested query in a reusable function:

private IQueryable<Order> LastSeasonOrders(Order order)
{
    return (from o in _context.Orders 
            where o.Year == order.Year - 1 && o.Product == order.Product
            select o);
}

which then lets me change the original query to:

from o in _context.Orders
where LastSeasonOrders(o).Count() > 100
select o;

This doesn’t work however with an exception saying that the method call cannot be translated to SQL when the query is run.

Any quick tips on the correct way to achieve this?

Answer 1

What about something like –

void Main()
{
    TypedDataContext _context = ...

    var query = 
        (
            from o in _context.Orders  
            where LastSeasonOrders(_context , o).Count() > 100 
            select o    
        );
     ...
 }      


public static Func<TypedDataContext, Order, IQueryable<Order>> 
     LastSeasonOrders = CompiledQuery.Compile
     (( TypedDataContext _context, Order order) =>

        from o in _context.Orders
        where o.Year == order.Year - 1 && o.Product == order.Product
        select o            
);          

?

It would be best to verify that the sql produced is the same as that produced by your original query.