Follow-up to Dmitry's patch for NH-2566: Make the rewrite more cautious by only doing it if the result operators in the outer query are identified as compatible for this procedure. Also add a test case more similar to the one in the bug report, plus with an order-by clause to test this likely combination.

Author: oskarb

src/NHibernate.Test/Linq/ByMethod/GroupByTests.cs

@@ -254,6 +254,24 @@ public void GroupByAndTake()
 			Assert.That(names.Count, Is.EqualTo(3));
 		}
 
+
+		[Test]
+		public void GroupByAndTake2()
+		{
+			//NH-2566
+			var results = (from o in db.Orders
+			               group o by o.Customer
+			               into g
+			               select g.Key.CustomerId)
+				.OrderBy(customerId => customerId)
+				.Skip(10)
+				.Take(10)
+				.ToList();
+			
+			Assert.That(results.Count, Is.EqualTo(10));
+		}
+
+
 		private static void CheckGrouping<TKey, TElement>(IEnumerable<IGrouping<TKey, TElement>> groupedItems, Func<TElement, TKey> groupBy)
 		{
 			var used = new HashSet<object>();

src/NHibernate/Linq/GroupBy/AggregatingGroupByRewriter.cs

@@ -42,11 +42,25 @@ public static void ReWrite(QueryModel queryModel)
 			}
 		}
 
+
+		private static readonly System.Type[] AcceptableOuterResultOperators = new[]
+			{
+				typeof (SkipResultOperator),
+				typeof (TakeResultOperator),
+			};
+
+
 		private static void FlattenSubQuery(SubQueryExpression subQueryExpression, QueryModel queryModel)
 		{
-			var groupBy = (GroupResultOperator) subQueryExpression.QueryModel.ResultOperators[0];
+			foreach (var resultOperator in queryModel.ResultOperators)
+			{
+				if (!AcceptableOuterResultOperators.Contains(resultOperator.GetType()))
+					throw new NotImplementedException("Cannot use group by with the "
+					                                  + resultOperator.GetType().Name + " result operator.");
+			}
 
-			// Move the result operator up 
+			// Move the result operator up.
+			var groupBy = (GroupResultOperator) subQueryExpression.QueryModel.ResultOperators[0];
 			queryModel.ResultOperators.Insert(0, groupBy);
 
 			for (int i = 0; i < queryModel.BodyClauses.Count; i++)