Update IIsEntityDecider to use ExpressionsHelper.TryGetMappedType

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 Animal FatherOrMother => Father ?? Mother;
+	}
 
     public abstract class Reptile : Animal
     {
@@ -30,4 +32,4 @@ public abstract class Mammal : Animal
     public class Dog : Mammal { }
 
     public class Cat : Mammal { }
-}
+}

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

@@ -289,6 +289,14 @@ public async Task CanSelectFirstElementFromChildCollectionAsync()
 			}
 		}
 
+		[Test]
+		public async Task CanSelectNotMappedEntityPropertyAsync()
+		{
+			var list = await (db.Animals.Where(o => o.Mother != null).Select(o => o.FatherOrMother.SerialNumber).ToListAsync());
+
+			Assert.That(list, Has.Count.GreaterThan(0));
+		}
+
 		[Test]
 		public async Task CanProjectWithCastAsync()
 		{

src/NHibernate.Test/Linq/SelectionTests.cs

@@ -328,6 +328,14 @@ public void CanSelectWrappedType()
 			Assert.IsTrue(query.ToArray().Length > 0);
 		}
 
+		[Test]
+		public void CanSelectNotMappedEntityProperty()
+		{
+			var list = db.Animals.Where(o => o.Mother != null).Select(o => o.FatherOrMother.SerialNumber).ToList();
+
+			Assert.That(list, Has.Count.GreaterThan(0));
+		}
+
 		[Test]
 		public void CanProjectWithCast()
 		{

src/NHibernate/Linq/ReWriters/AddJoinsReWriter.cs

@@ -1,18 +1,19 @@
 using System;
 using System.Collections.Specialized;
 using System.Linq;
+using System.Linq.Expressions;
 using NHibernate.Engine;
 using NHibernate.Linq.Clauses;
 using NHibernate.Linq.Visitors;
+using NHibernate.Util;
 using Remotion.Linq;
 using Remotion.Linq.Clauses;
 
 namespace NHibernate.Linq.ReWriters
 {
 	internal interface IIsEntityDecider
 	{
-		bool IsEntity(System.Type type);
-		bool IsIdentifier(System.Type type, string propertyName);
+		bool IsEntity(MemberExpression expression, out bool isIdentifier);
 	}
 
 	public class AddJoinsReWriter : NhQueryModelVisitorBase, IIsEntityDecider
@@ -77,11 +78,15 @@ public override void VisitJoinClause(JoinClause joinClause, QueryModel queryMode
 			_currentJoin = null;
 		}
 
+		// Since v5.3
+		[Obsolete("This method has no usages and will be removed in a future version")]
 		public bool IsEntity(System.Type type)
 		{
 			return _sessionFactory.GetImplementors(type.FullName).Any();
 		}
 
+		// Since v5.3
+		[Obsolete("This method has no usages and will be removed in a future version")]
 		public bool IsIdentifier(System.Type type, string propertyName)
 		{
 			var metadata = _sessionFactory.GetClassMetadata(type);
@@ -99,5 +104,14 @@ private void AddJoin(QueryModel queryModel, NhJoinClause joinClause)
 
 			queryModel.BodyClauses.Add(joinClause);
 		}
+
+		bool IIsEntityDecider.IsEntity(MemberExpression expression, out bool isIdentifier)
+		{
+			isIdentifier =
+				ExpressionsHelper.TryGetMappedType(_sessionFactory, expression, out var mappedType, out var entityPersister, out _, out var memberPath)
+				&& entityPersister?.IdentifierPropertyName == memberPath;
+
+			return mappedType?.IsEntityType == true;
+		}
 	}
 }

src/NHibernate/Linq/Visitors/MemberExpressionJoinDetector.cs

@@ -1,6 +1,7 @@
 using System.Collections;
 using System.Collections.Generic;
 using System.Linq.Expressions;
+using NHibernate.Engine;
 using NHibernate.Linq.Expressions;
 using NHibernate.Linq.ReWriters;
 using Remotion.Linq.Clauses;
@@ -39,7 +40,7 @@ protected override Expression VisitMember(MemberExpression expression)
 				return base.VisitMember(expression);
 			}
 
-			var isIdentifier = _isEntityDecider.IsIdentifier(expression.Expression.Type, expression.Member.Name);
+			var isEntity = _isEntityDecider.IsEntity(expression, out var isIdentifier);
 			if (isIdentifier)
 				_hasIdentifier = true;
 			if (!isIdentifier)
@@ -50,7 +51,7 @@ protected override Expression VisitMember(MemberExpression expression)
 			if (!isIdentifier)
 				_memberExpressionDepth--;
 
-			if (_isEntityDecider.IsEntity(expression.Type) &&
+			if (isEntity &&
 				((_requiresJoinForNonIdentifier && !_hasIdentifier) || _memberExpressionDepth > 0) &&
 				_joiner.CanAddJoin(expression))
 			{

src/NHibernate/Linq/Visitors/SelectClauseNominator.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Linq.Expressions;
+using NHibernate.Engine;
 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 ISessionFactoryImplementor _sessionFactory;
 
 		/// <summary>
 		/// The expression parts that can be converted to pure HQL.
@@ -35,6 +37,7 @@ class SelectClauseHqlNominator : RelinqExpressionVisitor
 		public SelectClauseHqlNominator(VisitorParameters parameters)
 		{
 			_functionRegistry = parameters.SessionFactory.Settings.LinqToHqlGeneratorsRegistry;
+			_sessionFactory = parameters.SessionFactory;
 		}
 
 		internal Expression Nominate(Expression expression)
@@ -168,8 +171,10 @@ private bool CanBeEvaluatedInHqlSelectStatement(Expression expression, bool proj
 				return projectConstantsInHql;
 			}
 
-			// Assume all is good
-			return true;
+			return !(expression is MemberExpression memberExpression) || // Assume all is good
+			       // Nominate only expressions that represent a mapped property or a translatable method call
+			       ExpressionsHelper.TryGetMappedType(_sessionFactory, expression, out _, out _, out _, out _) ||
+			       _functionRegistry.TryGetGenerator(memberExpression.Member, out _);
 		}
 
 		private static bool CanBeEvaluatedInHqlStatementShortcut(Expression expression)

src/NHibernate/Linq/Visitors/WhereJoinDetector.cs

@@ -2,6 +2,7 @@
 using System.Collections.Generic;
 using System.Linq;
 using System.Linq.Expressions;
+using NHibernate.Engine;
 using NHibernate.Linq.ReWriters;
 using Remotion.Linq.Clauses;
 using Remotion.Linq.Clauses.Expressions;
@@ -314,10 +315,7 @@ protected override Expression VisitMember(MemberExpression expression)
 				return base.VisitMember(expression);
 			}
 
-			var isIdentifier = _isEntityDecider.IsIdentifier(
-				expression.Expression.Type,
-				expression.Member.Name);
-
+			var isEntity = _isEntityDecider.IsEntity(expression, out var isIdentifier);
 			if (!isIdentifier)
 				_memberExpressionDepth++;
 
@@ -327,7 +325,7 @@ protected override Expression VisitMember(MemberExpression expression)
 				_memberExpressionDepth--;
 
 			ExpressionValues values = _values.Pop().Operation(pvs => pvs.MemberAccess(expression.Type));
-			if (_isEntityDecider.IsEntity(expression.Type))
+			if (isEntity)
 			{
 				// Don't add joins for things like a.B == a.C where B and C are entities.
 				// We only need to join B when there's something like a.B.D.