Fix conditional expressions not being translated into hql

src/NHibernate.Test/Async/Linq/SelectionTests.cs

@@ -270,6 +270,62 @@ public async Task CanSelectWithAnySubQueryAsync()
 			Assert.AreEqual(1, list.Count(t => !t.HasEntries));
 		}
 
+		[Test]
+		public async Task CanSelectConditionalAsync()
+		{
+			using (var sqlLog = new SqlLogSpy())
+			{
+				var q = await (db.Orders.Where(o => o.Customer.CustomerId == "test")
+						   .Select(o => o.ShippedTo.Contains("test") ? o.ShippedTo : o.Customer.CompanyName)
+						   .OrderBy(o => o)
+						   .Distinct()
+						   .ToListAsync());
+
+				Assert.That(FindAllOccurrences(sqlLog.GetWholeLog(), "case"), Is.EqualTo(2));
+			}
+
+			using (var sqlLog = new SqlLogSpy())
+			{
+				var q = await (db.Orders.Where(o => o.Customer.CustomerId == "test")
+						   .Select(o => o.OrderDate.HasValue ? o.OrderDate : o.ShippingDate)
+						   .FirstOrDefaultAsync());
+
+				Assert.That(FindAllOccurrences(sqlLog.GetWholeLog(), "case"), Is.EqualTo(1));
+			}
+
+			using (var sqlLog = new SqlLogSpy())
+			{
+				var q = await (db.Orders.Where(o => o.Customer.CustomerId == "test")
+						   .Select(o => new
+						   {
+							   Value = o.OrderDate.HasValue
+								   ? o.Customer.CompanyName
+								   : (o.ShippingDate.HasValue
+									? o.Shipper.CompanyName
+									: o.ShippedTo)
+						   })
+						   .FirstOrDefaultAsync());
+
+				var log = sqlLog.GetWholeLog();
+				Assert.That(FindAllOccurrences(log, "as col"), Is.EqualTo(1));
+			}
+		}
+
+		[Test]
+		public async Task CanSelectConditionalSubQueryAsync()
+		{
+			var orders = await (db.Customers
+			               .Select(c => new
+			               {
+				               Date = db.Orders.Where(o => o.Customer.CustomerId == c.CustomerId)
+				                        .Select(o => o.OrderDate.HasValue ? o.OrderDate : o.ShippingDate)
+				                        .First()
+			               })
+			               .ToListAsync());
+
+			Assert.That(orders, Has.Count.GreaterThan(0));
+		}
+
 		[Test, KnownBug("NH-3045")]
 		public async Task CanSelectFirstElementFromChildCollectionAsync()
 		{
@@ -458,5 +514,20 @@ public class Wrapper<T>
 			public T item;
 			public string message;
 		}
+
+		private int FindAllOccurrences(string source, string substring)
+		{
+			if (source == null)
+			{
+				return 0;
+			}
+			int n = 0, count = 0;
+			while ((n = source.IndexOf(substring, n, StringComparison.InvariantCulture)) != -1)
+			{
+				n += substring.Length;
+				++count;
+			}
+			return count;
+		}
 	}
 }

src/NHibernate.Test/Linq/SelectionTests.cs

@@ -299,6 +299,62 @@ public void CanSelectWithAggregateSubQuery()
 			Assert.AreEqual(4, timesheets[2].EntryCount);
 		}
 
+		[Test]
+		public void CanSelectConditional()
+		{
+			using (var sqlLog = new SqlLogSpy())
+			{
+				var q = db.Orders.Where(o => o.Customer.CustomerId == "test")
+						   .Select(o => o.ShippedTo.Contains("test") ? o.ShippedTo : o.Customer.CompanyName)
+						   .OrderBy(o => o)
+						   .Distinct()
+						   .ToList();
+
+				Assert.That(FindAllOccurrences(sqlLog.GetWholeLog(), "case"), Is.EqualTo(2));
+			}
+
+			using (var sqlLog = new SqlLogSpy())
+			{
+				var q = db.Orders.Where(o => o.Customer.CustomerId == "test")
+						   .Select(o => o.OrderDate.HasValue ? o.OrderDate : o.ShippingDate)
+						   .FirstOrDefault();
+
+				Assert.That(FindAllOccurrences(sqlLog.GetWholeLog(), "case"), Is.EqualTo(1));
+			}
+
+			using (var sqlLog = new SqlLogSpy())
+			{
+				var q = db.Orders.Where(o => o.Customer.CustomerId == "test")
+						   .Select(o => new
+						   {
+							   Value = o.OrderDate.HasValue
+								   ? o.Customer.CompanyName
+								   : (o.ShippingDate.HasValue
+									? o.Shipper.CompanyName
+									: o.ShippedTo)
+						   })
+						   .FirstOrDefault();
+
+				var log = sqlLog.GetWholeLog();
+				Assert.That(FindAllOccurrences(log, "as col"), Is.EqualTo(1));
+			}
+		}
+
+		[Test]
+		public void CanSelectConditionalSubQuery()
+		{
+			var orders = db.Customers
+			               .Select(c => new
+			               {
+				               Date = db.Orders.Where(o => o.Customer.CustomerId == c.CustomerId)
+				                        .Select(o => o.OrderDate.HasValue ? o.OrderDate : o.ShippingDate)
+				                        .First()
+			               })
+			               .ToList();
+
+			Assert.That(orders, Has.Count.GreaterThan(0));
+		}
+
 		[Test, KnownBug("NH-3045")]
 		public void CanSelectFirstElementFromChildCollection()
 		{
@@ -497,5 +553,20 @@ public class Wrapper<T>
 			public T item;
 			public string message;
 		}
+
+		private int FindAllOccurrences(string source, string substring)
+		{
+			if (source == null)
+			{
+				return 0;
+			}
+			int n = 0, count = 0;
+			while ((n = source.IndexOf(substring, n, StringComparison.InvariantCulture)) != -1)
+			{
+				n += substring.Length;
+				++count;
+			}
+			return count;
+		}
 	}
 }

src/NHibernate/Hql/Ast/HqlTreeNode.cs

@@ -257,6 +257,30 @@ internal HqlIdent(IASTFactory factory, System.Type type)
 					throw new NotSupportedException(string.Format("Don't currently support idents of type {0}", type.Name));
 			}
 		}
+
+		internal static bool SupportsType(System.Type type)
+		{
+			type = type.UnwrapIfNullable();
+			switch (System.Type.GetTypeCode(type))
+			{
+				case TypeCode.Boolean:
+				case TypeCode.Int16:
+				case TypeCode.Int32:
+				case TypeCode.Int64:
+				case TypeCode.Decimal:
+				case TypeCode.Single:
+				case TypeCode.DateTime:
+				case TypeCode.String:
+				case TypeCode.Double:
+					return true;
+				default:
+					return new[]
+					{
+						typeof(Guid),
+						typeof(DateTimeOffset)
+					}.Contains(type);
+			}
+		}
 	}
 
 	public class HqlRange : HqlStatement

src/NHibernate/Linq/Visitors/SelectClauseNominator.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Linq.Expressions;
+using NHibernate.Hql.Ast;
 using NHibernate.Linq.Functions;
 using NHibernate.Linq.Expressions;
 using NHibernate.Util;
@@ -15,6 +16,7 @@ namespace NHibernate.Linq.Visitors
 	class SelectClauseHqlNominator : RelinqExpressionVisitor
 	{
 		private readonly ILinqToHqlGeneratorsRegistry _functionRegistry;
+		private readonly VisitorParameters _parameters;
 
 		/// <summary>
 		/// The expression parts that can be converted to pure HQL.
@@ -34,6 +36,7 @@ class SelectClauseHqlNominator : RelinqExpressionVisitor
 
 		public SelectClauseHqlNominator(VisitorParameters parameters)
 		{
+			_parameters = parameters;
 			_functionRegistry = parameters.SessionFactory.Settings.LinqToHqlGeneratorsRegistry;
 		}
 
@@ -145,7 +148,7 @@ private bool CanBeEvaluatedInHqlSelectStatement(Expression expression, bool proj
 			}
 
 			// Constants will only be evaluated in HQL if they're inside a method call
-			if (expression.NodeType == ExpressionType.Constant)
+			if (expression is ConstantExpression constantExpression && _parameters.ConstantToParameterMap.TryGetValue(constantExpression, out _))
 			{
 				return projectConstantsInHql;
 			}
@@ -156,14 +159,17 @@ private bool CanBeEvaluatedInHqlSelectStatement(Expression expression, bool proj
 				return IsRegisteredFunction(expression);
 			}
 
-			if (expression.NodeType == ExpressionType.Conditional)
+			if (expression is ConditionalExpression conditionalExpression)
 			{
 				// Theoretically, any conditional that returns a CAST-able primitive should be constructable in HQL.
 				// The type needs to be CAST-able because HQL wraps the CASE clause in a CAST and only supports
 				// certain types (as defined by the HqlIdent constructor that takes a System.Type as the second argument).
-				// However, this may still not cover all cases, so to limit the nomination of conditional expressions,
-				// we will only consider those which are already getting constants projected into them.
-				return projectConstantsInHql;
+				return
+					HqlIdent.SupportsType(conditionalExpression.IfFalse.Type) &&
+					HqlCandidates.Contains(conditionalExpression.IfFalse) &&
+					HqlIdent.SupportsType(conditionalExpression.IfTrue.Type) &&
+					HqlCandidates.Contains(conditionalExpression.IfTrue) &&
+					HqlCandidates.Contains(conditionalExpression.Test);
 			}
 
 			// Assume all is good
@@ -175,4 +181,4 @@ private static bool CanBeEvaluatedInHqlStatementShortcut(Expression expression)
 			return expression is NhCountExpression;
 		}
 	}
-}
+}