Added unit test

Author: rjperes

src/NHibernate.Test/Linq/LinqQuerySamples.cs

@@ -897,6 +897,22 @@ into g
 			ObjectDumper.Write(q, 1);
 		}
 
+		[Test]
+		public void GroupTwoQueriesAndSum()
+		{
+			//NH-3534
+			var queryWithAggregation = from o1 in db.Orders
+						 from o2 in db.Orders
+						 where o1.Customer.CustomerId == o2.Customer.CustomerId && o1.OrderDate == o2.OrderDate
+						 group o1 by new { o1.Customer.CustomerId, o1.OrderDate } into g
+						 select new { CustomerId = g.Key.CustomerId, LastOrderDate = g.Max(x => x.OrderDate) };
+
+			var result = queryWithAggregation.ToList();
+
+			Assert.IsNotNull(result);
+			Assert.IsNotEmpty(result);
+		}
+
 		[Category("GROUP BY/HAVING")]
 		[Test(Description = "This sample uses a where clause after a group by clause " +
 							"to find all categories that have at least 10 products.")]