Question

I am trying to write a Linq query that results in a parent entity with one of the fields being a collection of entities of its related children. For example, I have a collection of all customers entities and a collection of all orders entities. The orders entities have a field called customerPK which contains the link to the related parent customer entity. I want to create a Linq query that joins the two collections and results in all the fields of the customer entity plus an additional field which is the collection object of all the related order entities for that specific customer entity.

Answer 1

Hopefully this should do the trick;

EDIT: Code updated to perform a Left Outer Join, based on this example; http://smehrozalam.wordpress.com/2009/06/10/c-left-outer-joins-with-linq/. This now includes Customers who have no orders.

var query = from c in customers
            join o in orders on c.ID equals o.CustomerPK into joined
            from j in joined.DefaultIfEmpty()
            group j by c into g
            select new { Customer = g.Key, Orders = g.Where(x => x != null) };

Note, the use of Where on the selection of the Orders grouping is so that null orders are filtered out at this point, instead of ending up with a grouping containing a single null Order for customers who don’t have orders.

Then some example usage;

foreach (var result in query)
{
    Console.WriteLine("{0} (ID={1})", result.Customer.Name, result.Customer.ID);
    foreach (var order in result.Orders)
    {
        Console.WriteLine(order.Description);
    }
}

This example results in an object with two fields, the Customer and then a group of related Orders, but there’s no reason why you can’t select the individual fields of your customer object in the query as you specified in your post.