NH-3797 - Use group by keys as first class HqlCandidates for select clause rewriting

We make the assumption that if the group by key is being sent to the DB for grouping then we can send the same expression to the DB for selecting.

Author: PleasantD

src/NHibernate/Linq/GroupBy/AggregatingGroupByRewriter.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using System.Linq.Expressions;
 using NHibernate.Linq.Clauses;
 using NHibernate.Linq.ReWriters;
 using NHibernate.Linq.Visitors;
@@ -42,9 +43,10 @@ public static class AggregatingGroupByRewriter
 				typeof (CacheableResultOperator)
 			};
 
-		public static void ReWrite(QueryModel queryModel)
+        public static IEnumerable<Expression> ReWrite(QueryModel queryModel)
 		{
-			var subQueryExpression = queryModel.MainFromClause.FromExpression as SubQueryExpression;
+            IEnumerable<Expression> groupByKeys = Enumerable.Empty<Expression>();
+            var subQueryExpression = queryModel.MainFromClause.FromExpression as SubQueryExpression;
 
 			if (subQueryExpression != null)
 			{
@@ -57,10 +59,17 @@ public static void ReWrite(QueryModel queryModel)
 					var groupBy = operators[0] as GroupResultOperator;
 					if (groupBy != null)
 					{
-						FlattenSubQuery(queryModel, subQueryExpression.QueryModel, groupBy);
+                        if (groupBy.KeySelector is NewExpression)
+                            groupByKeys = (groupBy.KeySelector as NewExpression).Arguments;
+                        else if (groupBy.KeySelector is NewArrayExpression)
+                            groupByKeys = (groupBy.KeySelector as NewArrayExpression).Expressions;
+                        else
+                            groupByKeys = new[] { groupBy.KeySelector };
+                        FlattenSubQuery(queryModel, subQueryExpression.QueryModel, groupBy);
 					}
 				}
 			}
+		    return groupByKeys;
 		}
 
 		private static void FlattenSubQuery(QueryModel queryModel, QueryModel subQueryModel, GroupResultOperator groupBy)

src/NHibernate/Linq/Visitors/QueryModelVisitor.cs

@@ -1,4 +1,6 @@
 using System;
+using System.Collections;
+using System.Collections.Generic;
 using System.Linq.Expressions;
 using NHibernate.Hql.Ast;
 using NHibernate.Linq.Clauses;
@@ -32,7 +34,7 @@ public static ExpressionToHqlTranslationResults GenerateHqlQuery(QueryModel quer
 			NonAggregatingGroupByRewriter.ReWrite(queryModel);
 
 			// Rewrite aggregate group-by statements
-			AggregatingGroupByRewriter.ReWrite(queryModel);
+			var groupByKeys = AggregatingGroupByRewriter.ReWrite(queryModel);
 
 			// Rewrite aggregating group-joins
 			AggregatingGroupJoinRewriter.ReWrite(queryModel);
@@ -74,7 +76,11 @@ public static ExpressionToHqlTranslationResults GenerateHqlQuery(QueryModel quer
 			// Identify and name query sources
 			QuerySourceIdentifier.Visit(parameters.QuerySourceNamer, queryModel);
 
-			var visitor = new QueryModelVisitor(parameters, root, queryModel) { RewrittenOperatorResult = result };
+			var visitor = new QueryModelVisitor(parameters, root, queryModel)
+			{
+			    RewrittenOperatorResult = result,
+                GroupByKeys = groupByKeys,
+			};
 			visitor.Visit();
 
 			return visitor._hqlTree.GetTranslation();
@@ -94,6 +100,8 @@ public static ExpressionToHqlTranslationResults GenerateHqlQuery(QueryModel quer
 
 		public ResultOperatorRewriterResult RewrittenOperatorResult { get; private set; }
 
+        public IEnumerable<Expression> GroupByKeys { get; private set; }
+
 		static QueryModelVisitor()
 		{
 			// TODO - reflection to build map
@@ -230,7 +238,7 @@ public override void VisitSelectClause(SelectClause selectClause, QueryModel que
 		{
 			CurrentEvaluationType = selectClause.GetOutputDataInfo();
 
-			var visitor = new SelectClauseVisitor(typeof(object[]), VisitorParameters);
+			var visitor = new SelectClauseVisitor(typeof(object[]), VisitorParameters, GroupByKeys);
 
 			visitor.Visit(selectClause.Selector);
 

src/NHibernate/Linq/Visitors/SelectClauseVisitor.cs

@@ -18,11 +18,12 @@ public class SelectClauseVisitor : ExpressionTreeVisitor
 		private List<HqlExpression> _hqlTreeNodes = new List<HqlExpression>();
 		private readonly HqlGeneratorExpressionTreeVisitor _hqlVisitor;
 
-		public SelectClauseVisitor(System.Type inputType, VisitorParameters parameters)
+		public SelectClauseVisitor(System.Type inputType, VisitorParameters parameters, IEnumerable<Expression> groupByKeys)
 		{
 			_inputParameter = Expression.Parameter(inputType, "input");
 			_parameters = parameters;
 			_hqlVisitor = new HqlGeneratorExpressionTreeVisitor(_parameters);
+            _hqlNodes = new HashSet<Expression>(groupByKeys);
 		}
 
 		public LambdaExpression ProjectionExpression { get; private set; }
@@ -43,7 +44,7 @@ public void Visit(Expression expression)
 			// Find the sub trees that can be expressed purely in HQL
 			var nominator = new SelectClauseHqlNominator(_parameters);
 			nominator.Visit(expression);
-			_hqlNodes = nominator.HqlCandidates;
+			_hqlNodes.UnionWith(nominator.HqlCandidates);
 
 			// Linq2SQL ignores calls to local methods. Linq2EF seems to not support
 			// calls to local methods at all. For NHibernate we support local methods,