Improve select clause transformation for Linq provider

Author: maca88

src/NHibernate.DomainModel/Northwind/Entities/Animal.cs

@@ -12,7 +12,9 @@ public class Animal
         public virtual Animal Father { get; set; }
         public virtual IList<Animal> Children { get; set; }
         public virtual string SerialNumber { get; set; }
-    }
+		public virtual string FatherSerialNumber => Father?.SerialNumber;
+		public virtual bool HasFather => Father != null;
+	}
 
     public abstract class Reptile : Animal
     {
@@ -30,4 +32,4 @@ public abstract class Mammal : Animal
     public class Dog : Mammal { }
 
     public class Cat : Mammal { }
-}
+}

src/NHibernate.DomainModel/Northwind/Entities/UserComponent.cs

@@ -5,10 +5,17 @@ public class UserComponent
         public string Property1 { get; set; }
         public string Property2 { get; set; }
         public UserComponent2 OtherComponent { get; set; }
+
+		public string Property3 => $"{Property1}{Property2}";
+
     }
 
     public class UserComponent2
     {
         public string OtherProperty1 { get; set; }
+
+		public string OtherProperty2 { get; set; }
+
+		public string OtherProperty3 => OtherProperty1;
     }
-}
+}

src/NHibernate.DomainModel/Northwind/Mappings/User.hbm.xml

@@ -30,6 +30,7 @@
 
       <component name="OtherComponent" class="UserComponent2">
         <property name="OtherProperty1" type="AnsiString" />
+        <property name="OtherProperty2" type="AnsiString" formula="OtherProperty1"/>
       </component>
     </component>
   </class>

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

@@ -4,6 +4,7 @@
 using System.Linq;
 using System.Linq.Expressions;
 using System.Reflection;
+using NHibernate.Engine;
 using NHibernate.Hql.Ast;
 using NHibernate.Linq.Visitors;
 using NHibernate.Util;
@@ -68,6 +69,23 @@ public LengthGenerator()
 
 		public override HqlTreeNode BuildHql(MemberInfo member, Expression expression, HqlTreeBuilder treeBuilder, IHqlExpressionVisitor visitor)
 		{
+			if (visitor.SessionFactory is ISessionFactoryImplementor sessionFactory)
+			{
+				var sqlFunction = sessionFactory.SQLFunctionRegistry.FindSQLFunction("length");
+				if (sqlFunction == null)
+				{
+					throw new InvalidOperationException("Sql function length is not supported for the current dialect.");
+				}
+
+				var returnType = sqlFunction.ReturnType(NHibernateUtil.Int32, sessionFactory);
+				if (!NHibernateUtil.Int32.Equals(returnType))
+				{
+					return treeBuilder.Cast(
+						treeBuilder.MethodCall("length", visitor.Visit(expression).AsExpression()),
+						typeof(int));
+				}
+			}
+
 			return treeBuilder.MethodCall("length", visitor.Visit(expression).AsExpression());
 		}
 	}

src/NHibernate.Test/Linq/SelectionTests.cs

@@ -224,7 +224,7 @@ private static Expression GetIdentifier(ISessionFactory sessionFactory, Expressi
 
 			var classMetadata = sessionFactory.GetClassMetadata(expression.Type);
 			if (classMetadata == null)
-				return Expression.Constant(null);
+				return null;
 
             var propertyName=classMetadata.IdentifierPropertyName;
             NHibernate.Type.EmbeddedComponentType componentType;

src/NHibernate/Linq/Functions/StringGenerator.cs

@@ -23,7 +23,11 @@ public SelectClauseRewriter(Expression parameter, ICollection<ExpressionHolder>
 			this.expressions = expressions;
 			this.parameter = parameter;
 			this.tuple = tuple;
-			this.expressions.Add(new ExpressionHolder { Expression = expression, Tuple = tuple }); //ID placeholder
+			if (expression != null)
+			{
+				this.expressions.Add(new ExpressionHolder { Expression = expression, Tuple = tuple }); //ID placeholder
+			}
+
 			_dictionary = dictionary;
 		}
 
@@ -59,4 +63,4 @@ private Expression AddAndConvertExpression(Expression expression)
 				expression.Type);
 		}
 	}
-}
+}