NH-3474 - Fix support of GroupBy with constant keys

* Remove the GROUP BY clause entirely if all keys are constant
* Re-write the GROUP BY clause using only non-constant keys
* Adjust the ExpressionKeyVisitor so that constants in GroupBy are not replaced with parameter
* Fix ExpressionKeyVisitor handling of constant arrays to show initialization values

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

@@ -549,6 +549,100 @@ public void GroupByComputedValueInObjectArray()
 			Assert.AreEqual(830, orderGroups.Sum(g => g.Count));
 		}
 
+		[Test(Description = "NH-3474")]
+		public void GroupByConstant()
+		{
+			var totals = db.Orders.GroupBy(o => 1).Select(g => new { Key = g.Key, Count = g.Count(), Sum = g.Sum(x => x.Freight) }).ToList();
+			Assert.That(totals.Count, Is.EqualTo(1));
+			Assert.That(totals, Has.All.With.Property("Key").EqualTo(1));
+		}
+
+		[Test(Description = "NH-3474")]
+		public void GroupByConstantAnonymousType()
+		{
+			var totals = db.Orders.GroupBy(o => new { A = 1 }).Select(g => new { Key = g.Key, Count = g.Count(), Sum = g.Sum(x => x.Freight) }).ToList();
+			Assert.That(totals.Count, Is.EqualTo(1));
+			Assert.That(totals, Has.All.With.Property("Key").With.Property("A").EqualTo(1));
+		}
+
+		[Test(Description = "NH-3474")]
+		public void GroupByConstantArray()
+		{
+			var totals = db.Orders.GroupBy(o => new object[] { 1 }).Select(g => new { Key = g.Key, Count = g.Count(), Sum = g.Sum(x => x.Freight) }).ToList();
+			Assert.That(totals.Count, Is.EqualTo(1));
+			Assert.That(totals, Has.All.With.Property("Key").EqualTo(new object[] { 1 }));
+		}
+
+		[Test(Description = "NH-3474")]
+		public void GroupByKeyWithConstantInAnonymousType()
+		{
+			var totals = db.Orders.GroupBy(o => new { A = 1, B = o.Shipper.ShipperId }).Select(g => new { Key = g.Key, Count = g.Count(), Sum = g.Sum(x => x.Freight) }).ToList();
+			Assert.That(totals.Count, Is.EqualTo(3));
+			Assert.That(totals, Has.All.With.Property("Key").With.Property("A").EqualTo(1));
+		}
+
+		[Test(Description = "NH-3474")]
+		public void GroupByKeyWithConstantInArray()
+		{
+			var totals = db.Orders.GroupBy(o => new[] { 1, o.Shipper.ShipperId }).Select(g => new { Key = g.Key, Count = g.Count(), Sum = g.Sum(x => x.Freight) }).ToList();
+			Assert.That(totals.Count, Is.EqualTo(3));
+			Assert.That(totals, Has.All.With.Property("Key").Contains(1));
+		}
+
+		private int constKey;
+		[Test(Description = "NH-3474")]
+		public void GroupByKeyWithConstantFromVariable()
+		{
+			constKey = 1;
+			var q1 = db.Orders.GroupBy(o => constKey).Select(g => new {Key = g.Key, Count = g.Count(), Sum = g.Sum(x => x.Freight)});
+			var q2 = db.Orders.GroupBy(o => new {A = constKey}).Select(g => new {Key = g.Key, Count = g.Count(), Sum = g.Sum(x => x.Freight)});
+			var q3 = db.Orders.GroupBy(o => new object[] {constKey}).Select(g => new {Key = g.Key, Count = g.Count(), Sum = g.Sum(x => x.Freight)});
+			var q4 = db.Orders.GroupBy(o => new {A = constKey, B = o.Shipper.ShipperId}).Select(g => new {Key = g.Key, Count = g.Count(), Sum = g.Sum(x => x.Freight)});
+			var q5 = db.Orders.GroupBy(o => new[] {constKey, o.Shipper.ShipperId}).Select(g => new {Key = g.Key, Count = g.Count(), Sum = g.Sum(x => x.Freight)});
+
+			var r1_1 = q1.ToList();
+			Assert.That(r1_1.Count, Is.EqualTo(1));
+			Assert.That(r1_1, Has.All.With.Property("Key").EqualTo(1));
+
+			var r2_1 = q2.ToList();
+			Assert.That(r2_1.Count, Is.EqualTo(1));
+			Assert.That(r2_1, Has.All.With.Property("Key").With.Property("A").EqualTo(1));
+
+			var r3_1 = q3.ToList();
+			Assert.That(r3_1.Count, Is.EqualTo(1));
+			Assert.That(r3_1, Has.All.With.Property("Key").EqualTo(new object[] { 1 }));
+
+			var r4_1 = q4.ToList();
+			Assert.That(r4_1.Count, Is.EqualTo(3));
+			Assert.That(r4_1, Has.All.With.Property("Key").With.Property("A").EqualTo(1));
+
+			var r5_1 = q5.ToList();
+			Assert.That(r5_1.Count, Is.EqualTo(3));
+			Assert.That(r5_1, Has.All.With.Property("Key").Contains(1));
+
+			constKey = 2;
+
+			var r1_2 = q1.ToList();
+			Assert.That(r1_2.Count, Is.EqualTo(1));
+			Assert.That(r1_2, Has.All.With.Property("Key").EqualTo(2));
+
+			var r2_2 = q2.ToList();
+			Assert.That(r2_2.Count, Is.EqualTo(1));
+			Assert.That(r2_2, Has.All.With.Property("Key").With.Property("A").EqualTo(2));
+
+			var r3_2 = q3.ToList();
+			Assert.That(r3_2.Count, Is.EqualTo(1));
+			Assert.That(r3_2, Has.All.With.Property("Key").EqualTo(new object[] { constKey }));
+
+			var r4_2 = q4.ToList();
+			Assert.That(r4_2.Count, Is.EqualTo(3));
+			Assert.That(r4_2, Has.All.With.Property("Key").With.Property("A").EqualTo(2));
+
+			var r5_2 = q5.ToList();
+			Assert.That(r5_2.Count, Is.EqualTo(3));
+			Assert.That(r5_2, Has.All.With.Property("Key").Contains(2));
+		}
+
 		[Test(Description = "NH-3801")]
 		public void GroupByComputedValueWithJoinOnObject()
 		{

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;
@@ -58,6 +59,7 @@ public static void ReWrite(QueryModel queryModel)
 					if (groupBy != null)
 					{
 						FlattenSubQuery(queryModel, subQueryExpression.QueryModel, groupBy);
+						RemoveCostantGroupByKeys(queryModel, groupBy);
 					}
 				}
 			}
@@ -101,5 +103,22 @@ private static void FlattenSubQuery(QueryModel queryModel, QueryModel subQueryMo
 			// Replace the outer query source
 			queryModel.MainFromClause = subQueryModel.MainFromClause;
 		}
+
+		private static void RemoveCostantGroupByKeys(QueryModel queryModel, GroupResultOperator groupBy)
+		{
+			var keys = groupBy.ExtractKeyExpressions().Where(x => !(x is ConstantExpression)).ToList();
+
+			if (!keys.Any())
+			{
+				// Remove the Group By clause completely if all the keys are constant (redundant)
+				queryModel.ResultOperators.Remove(groupBy);
+			}
+			else
+			{
+				// Re-write the KeySelector as an object array of the non-constant keys
+				// This should be safe because we've already re-written the select clause using the original keys
+				groupBy.KeySelector = Expression.NewArrayInit(typeof (object), keys.Select(x => x.Type.IsValueType ? Expression.Convert(x, typeof(object)) : x));
+			}
+		}
 	}
 }

src/NHibernate/Linq/GroupResultOperatorExtensions.cs

@@ -0,0 +1,22 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Linq.Expressions;
+using System.Text;
+using NHibernate.Util;
+using Remotion.Linq.Clauses.ResultOperators;
+
+namespace NHibernate.Linq
+{
+	internal static class GroupResultOperatorExtensions
+	{
+		public static IEnumerable<Expression> ExtractKeyExpressions(this GroupResultOperator groupResult)
+		{
+			if (groupResult.KeySelector is NewExpression)
+				return (groupResult.KeySelector as NewExpression).Arguments;
+			if (groupResult.KeySelector is NewArrayExpression)
+				return (groupResult.KeySelector as NewArrayExpression).Expressions;
+			return new [] { groupResult.KeySelector };
+		}
+	}
+}

src/NHibernate/Linq/Visitors/ExpressionKeyVisitor.cs

@@ -1,3 +1,4 @@
+using System;
 using System.Collections;
 using System.Collections.Generic;
 using System.Linq;
@@ -92,9 +93,22 @@ protected override Expression VisitConstantExpression(ConstantExpression express
 			else
 			{
 				if (expression.Value == null)
+				{
 					_string.Append("NULL");
+				}
+				else if (expression.Type.IsArray)
+				{
+					// Const arrays all look the same (they just display the type of array and not the initializer contents
+					// Since the contents might be different, we need to include those as well so we don't use a cached query plan by mistake
+					_string.Append(expression.Value);
+					_string.Append(" {");
+					_string.Append(String.Join(",", (object[]) expression.Value));
+					_string.Append("}");
+				}
 				else
+				{
 					_string.Append(expression.Value);
+				}
 			}
 
 			return base.VisitConstantExpression(expression);
@@ -142,6 +156,7 @@ protected override Expression VisitMethodCallExpression(MethodCallExpression exp
 				case "Single":
 				case "SingleOrDefault":
 				case "Select":
+				case "GroupBy":
 					insideSelectClause = true;
 					break;
 				default:

src/NHibernate/NHibernate.csproj

@@ -299,6 +299,7 @@
     <Compile Include="Linq\Functions\EqualsGenerator.cs" />
     <Compile Include="Linq\GroupBy\KeySelectorVisitor.cs" />
     <Compile Include="Linq\GroupBy\PagingRewriter.cs" />
+    <Compile Include="Linq\GroupResultOperatorExtensions.cs" />
     <Compile Include="Linq\NestedSelects\NestedSelectDetector.cs" />
     <Compile Include="Linq\NestedSelects\Tuple.cs" />
     <Compile Include="Linq\NestedSelects\SelectClauseRewriter.cs" />