Question

using Entity Framework 4.1 is it possible to perform a left outer join with an IEnumerable?

For example, lets say i want to count the number of orders for each month.

var range = Enumerable.Range(1,6).AsQueryable();
var result = range
               .GroupJoin(context.Orders, i => i, o => o.Month, new 
               {
                   Month = i, 
                   Count = m.Count()
               });

Which would produce the following

Month, Count    
1, 0
2, 0
3, 10
4, 20
5, 0
6, 0

Answer 1

Only if you call context.Orders.ToList() and load all orders to your application. IEnumerable is in your application memory and context.Orders represent data in your database. You cannot join data in database with data in memory of your application. Converting IEnumerable by AsQueryable will not put it to database.

To do your requirement directly with ADO.NET and SQL you must either create and fill temporary table and perform a database join or create stored procedure with table valued parameter to perform database join. Neither technique is supported by EF.