Add support for select after nested group by

Author: hazzik

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

@@ -841,7 +841,7 @@ public void GroupByComputedValueFromNestedObjectSelect()
 		}
 
 		[Test(Description="GH-3076")]
-		public void NestedGroupBy()
+		public void NestedNonAggregateGroupBy()
 		{
 			var list = db.OrderLines
 				.GroupBy(x => new { x.Order.OrderId, x.Product.ProductId }) // this works fine
@@ -851,6 +851,18 @@ public void NestedGroupBy()
 			Assert.That(list, Has.Count.EqualTo(77));
 		}
 
+		[Test(Description="GH-3076")]
+		public void NestedNonAggregateGroupBySelect()
+		{
+			var list = db.OrderLines
+				.GroupBy(x => new { x.Order.OrderId, x.Product.ProductId }) // this works fine
+				.GroupBy(x => x.Key.ProductId) // exception: "A recognition error occurred"
+				.Select(x => new { ProductId = x })
+				.ToList();
+
+			Assert.That(list, Has.Count.EqualTo(77));
+		}
+
 		private static void CheckGrouping<TKey, TElement>(IEnumerable<IGrouping<TKey, TElement>> groupedItems, Func<TElement, TKey> groupBy)
 		{
 			var used = new HashSet<object>();

src/NHibernate/Linq/GroupBy/NonAggregatingGroupByRewriter.cs

@@ -18,21 +18,19 @@ public static void ReWrite(QueryModel queryModel)
 			    && queryModel.ResultOperators.All(r => r is GroupResultOperator)
 			    && IsNonAggregatingGroupBy(queryModel))
 			{
-				var source = queryModel.SelectClause.Selector;
-				for (var index = 0; index < queryModel.ResultOperators.Count; index++)
+				for (var i = 0; i < queryModel.ResultOperators.Count; i++)
 				{
-					var r = (GroupResultOperator) queryModel.ResultOperators[index];
-					queryModel.ResultOperators[index] = new NonAggregatingGroupBy(r);
-					source = new QuerySourceReferenceExpression(r);
+					var r = (GroupResultOperator) queryModel.ResultOperators[i];
+					queryModel.ResultOperators[i] = new NonAggregatingGroupBy(r);
 				}
 
 				return;
 			}
 
-			if ((queryModel.MainFromClause.FromExpression is SubQueryExpression subQueryExpression) 
-				&& (subQueryExpression.QueryModel.ResultOperators.Count == 1) 
-				&& (subQueryExpression.QueryModel.ResultOperators[0] is GroupResultOperator) 
-				&& (IsNonAggregatingGroupBy(queryModel)))
+			if (queryModel.MainFromClause.FromExpression is SubQueryExpression subQueryExpression 
+				&& subQueryExpression.QueryModel.ResultOperators.Count > 0 
+				&& subQueryExpression.QueryModel.ResultOperators.All(r => r is GroupResultOperator) 
+				&& IsNonAggregatingGroupBy(queryModel))
 			{
 				FlattenSubQuery(subQueryExpression, queryModel);
 			}
@@ -62,7 +60,11 @@ private static void FlattenSubQuery(SubQueryExpression subQueryExpression, Query
 				throw new NotImplementedException();
 			}
 
-			queryModel.ResultOperators.Add(new NonAggregatingGroupBy((GroupResultOperator) subQueryModel.ResultOperators[0]));
+			for (var i = 0; i < subQueryModel.ResultOperators.Count; i++)
+			{
+				queryModel.ResultOperators.Add(new NonAggregatingGroupBy((GroupResultOperator) subQueryModel.ResultOperators[i]));
+			}
+
 			queryModel.ResultOperators.Add(clientSideSelect);
 		}