Question

Is it possible to return multiple entities from a LinqToSql (or EF) query (the query is inside a method) so that the results will still be composable ?

Something like this:

public IQueryable<KeyValuePair<Customer, Product>> GetCustomerEntities()
{
    return
    (
        from    customer in this.Context.Customers
                join
                product in this.Context.Products on customer.ID equals product.CustomerID       
        select  new KeyValuePair<Customer, Product>(customer, product)
    );
}

Then I want to use the result of this method to further compose the query like:

this.GetCustomerEntities().Where(e => e.Key.Name == "my customer")

The method above compiles but it can’t be executed by LinqToSql because it can’t convert KeyValuePair to SQL, which is the expected behavior.

Is it possible to achieve this somehow ?

Answer 1

The problem here is that the runtime can’t see that passing customer into the constructor parameter is the same thing as looking at e.Key, since those two things are not obviously the same. You might try creating your own POCO type with getter and setter, i.e.

select new CustomerProduct { Customer = customer, Product = product }

and using that instead of KeyValuePair<Customer,Product>.

Inside a single method, an anonymous type would be the obvious choice.