Fix nullable entity projection in hql (#2809)

Fixes #2772

Author: bahusoid

src/NHibernate.Test/Async/Hql/EntityJoinHqlTest.cs

@@ -9,11 +9,13 @@
 
 
 using System;
+using System.Linq;
 using System.Text.RegularExpressions;
 using NHibernate.Cfg.MappingSchema;
 using NHibernate.Mapping.ByCode;
 using NHibernate.Test.Hql.EntityJoinHqlTestEntities;
 using NUnit.Framework;
+using NHibernate.Linq;
 
 namespace NHibernate.Test.Hql
 {
@@ -287,6 +289,24 @@ public async Task NullableOneToOneFetchQueryIsNotAffected2Async()
 			}
 		}
 
+		[Test(Description = "GH-2772")]
+		public async Task NullableEntityProjectionAsync()
+		{
+			using (var session = OpenSession())
+			using (session.BeginTransaction())
+			{
+				var nullableOwner1 = new NullableOwner {Name = "1", ManyToOne = await (session.LoadAsync<OneToOneEntity>(Guid.NewGuid()))};
+				var nullableOwner2 = new NullableOwner {Name = "2"};
+				await (session.SaveAsync(nullableOwner1));
+				await (session.SaveAsync(nullableOwner2));
+
+				var fullList = await (session.Query<NullableOwner>().Select(x => new {x.Name, ManyToOneId = (Guid?) x.ManyToOne.Id}).ToListAsync());
+				var withValidManyToOneList = await (session.Query<NullableOwner>().Where(x => x.ManyToOne != null).Select(x => new {x.Name, ManyToOneId = (Guid?) x.ManyToOne.Id}).ToListAsync());
+				Assert.That(fullList.Count, Is.EqualTo(2));
+				Assert.That(withValidManyToOneList.Count, Is.EqualTo(0));
+			}
+		}
+
 		[Test]
 		public async Task EntityJoinWithEntityComparisonInWithClausShouldNotAddJoinAsync()
 		{

src/NHibernate.Test/Hql/EntityJoinHqlTest.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Linq;
 using System.Text.RegularExpressions;
 using NHibernate.Cfg.MappingSchema;
 using NHibernate.Mapping.ByCode;
@@ -276,6 +277,24 @@ public void NullableOneToOneFetchQueryIsNotAffected2()
 			}
 		}
 
+		[Test(Description = "GH-2772")]
+		public void NullableEntityProjection()
+		{
+			using (var session = OpenSession())
+			using (session.BeginTransaction())
+			{
+				var nullableOwner1 = new NullableOwner {Name = "1", ManyToOne = session.Load<OneToOneEntity>(Guid.NewGuid())};
+				var nullableOwner2 = new NullableOwner {Name = "2"};
+				session.Save(nullableOwner1);
+				session.Save(nullableOwner2);
+
+				var fullList = session.Query<NullableOwner>().Select(x => new {x.Name, ManyToOneId = (Guid?) x.ManyToOne.Id}).ToList();
+				var withValidManyToOneList = session.Query<NullableOwner>().Where(x => x.ManyToOne != null).Select(x => new {x.Name, ManyToOneId = (Guid?) x.ManyToOne.Id}).ToList();
+				Assert.That(fullList.Count, Is.EqualTo(2));
+				Assert.That(withValidManyToOneList.Count, Is.EqualTo(0));
+			}
+		}
+
 		[Test]
 		public void EntityJoinWithEntityComparisonInWithClausShouldNotAddJoin()
 		{

src/NHibernate/Hql/Ast/ANTLR/Tree/DotNode.cs

@@ -389,7 +389,7 @@ private void DereferenceEntity(EntityType entityType, bool implicitJoin, string
 			//For nullable entity comparisons we always need to add join (like not constrained one-to-one or not-found ignore associations)
 			//NOTE: This fix is not fully correct. It doesn't work for comparisons with null (where e.OneToOneProp is null)
 			// as by default implicit join is generated and to work propelry left join is required (see GH-2611)
-			bool comparisonWithNullableEntity = false;
+			bool comparisonWithNullableEntity = entityType.IsNullable && Walker.IsComparativeExpressionClause;
 
 			if ( IsDotNode( parent ) ) 
 			{
@@ -398,7 +398,7 @@ private void DereferenceEntity(EntityType entityType, bool implicitJoin, string
 				// entity's PK (because 'our' table would know the FK).
 				parentAsDotNode = ( DotNode ) parent;
 				property = parentAsDotNode._propertyName;
-				joinIsNeeded = generateJoin && ((Walker.IsSelectStatement && entityType.IsNullable) || !IsReferenceToPrimaryKey( parentAsDotNode._propertyName, entityType ));
+				joinIsNeeded = generateJoin && ((Walker.IsSelectStatement && comparisonWithNullableEntity) || !IsReferenceToPrimaryKey( parentAsDotNode._propertyName, entityType ));
 			}
 			else if ( ! Walker.IsSelectStatement ) 
 			{
@@ -411,7 +411,6 @@ private void DereferenceEntity(EntityType entityType, bool implicitJoin, string
 			}
 			else
 			{
-				comparisonWithNullableEntity = (Walker.IsComparativeExpressionClause && entityType.IsNullable);
 				joinIsNeeded = generateJoin || (Walker.IsInSelect && !Walker.IsInCase) || (Walker.IsInFrom && !Walker.IsComparativeExpressionClause)
 				               || comparisonWithNullableEntity;
 			}