Improve the fix attempt

Author: fredericDelaporte

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

@@ -403,19 +403,20 @@ private void DereferenceEntity(EntityType entityType, bool implicitJoin, string
 			bool joinIsNeeded;
 
 			// For nullable entity comparisons we always need to add join (like not constrained one-to-one or not-found ignore associations).
-			// For property-ref associations, we also need this unless finding a way in the IdentNode for the other hand of the comparison
-			// to detect it should yield the property-ref columns instead of the primary key columns.
-			bool comparisonWithNullableEntityOrThroughPropertyRef = Walker.IsComparativeExpressionClause
-				&& (entityType.IsNullable || entityType.IsUniqueKeyReference);
+			var comparisonWithNullableEntity = Walker.IsComparativeExpressionClause && entityType.IsNullable;
+			// For property-ref association comparison, we also need to join unless finding a way in the node for the other hand of the comparison
+			// to detect it should yield the property-ref columns instead of the primary key columns. And if the other hand is an association too,
+			// it may be a reference to the primary key, so we would need to join anyway.
+			var comparisonThroughPropertyRef = Walker.IsComparativeExpressionClause && !entityType.IsReferenceToPrimaryKey;
 
-			if ( IsDotNode( parent ) ) 
+			if (IsDotNode(parent))
 			{
 				// our parent is another dot node, meaning we are being further dereferenced.
 				// thus we need to generate a join unless the parent refers to the associated
 				// entity's PK (because 'our' table would know the FK).
 				parentAsDotNode = ( DotNode ) parent;
 				property = parentAsDotNode._propertyName;
-				joinIsNeeded = generateJoin && ((Walker.IsSelectStatement && comparisonWithNullableEntityOrThroughPropertyRef) || !IsReferenceToPrimaryKey( parentAsDotNode._propertyName, entityType ));
+				joinIsNeeded = generateJoin && ((Walker.IsSelectStatement && comparisonWithNullableEntity) || !IsReferenceToPrimaryKey( parentAsDotNode._propertyName, entityType ));
 			}
 			else if ( ! Walker.IsSelectStatement ) 
 			{
@@ -429,14 +430,15 @@ private void DereferenceEntity(EntityType entityType, bool implicitJoin, string
 			else
 			{
 				joinIsNeeded = generateJoin || (Walker.IsInSelect && !Walker.IsInCase) || (Walker.IsInFrom && !Walker.IsComparativeExpressionClause)
-				               || comparisonWithNullableEntityOrThroughPropertyRef;
+					|| comparisonWithNullableEntity || comparisonThroughPropertyRef;
 			}
 
 			if ( joinIsNeeded )
 			{
-				var forceLeftJoin = comparisonWithNullableEntityOrThroughPropertyRef && !IsCorrelatedSubselect;
+				// Subselect queries use theta style joins, which cannot be forced to left outer joins.
+				var forceLeftJoin = comparisonWithNullableEntity && !IsCorrelatedSubselect;
 				DereferenceEntityJoin(classAlias, entityType, implicitJoin, parent, forceLeftJoin);
-				if (comparisonWithNullableEntityOrThroughPropertyRef)
+				if (comparisonWithNullableEntity || comparisonThroughPropertyRef)
 				{
 					_columns = FromElement.GetIdentityColumns();
 				}