Fix fetching many-to-many with subclasses

Test case

Author: bahusoid

src/NHibernate.Test/NHSpecificTest/NH2174/Entity.cs

@@ -86,6 +86,7 @@ public override int GetHashCode()
 		private int _id_Doc;
 		private int _id_base;
 		public virtual IList<DocumentDetailDocument> RefferedDetails { get; set; } = new List<DocumentDetailDocument>();
+		public virtual IList<DocumentDetailDocument> RefferedDetailsManyToMany { get; set; } = new List<DocumentDetailDocument>();
 
 		public int Id_Doc
 		{

src/NHibernate.Test/NHSpecificTest/NH2174/Fixture.cs

@@ -14,8 +14,10 @@ protected override void OnSetUp()
 			{
 				var doc = new Document {Id_Base = 1, Id_Doc = 2};
 				session.Save(doc);
-				session.Save(new DocumentDetailDocument {Id_Base = 1, Id_Doc = 2, Id_Item = 1, ReferencedDocument = doc});
+				var detail = new DocumentDetailDocument {Id_Base = 1, Id_Doc = 2, Id_Item = 1, ReferencedDocument = doc};
+				session.Save(detail);
 
+				doc.RefferedDetailsManyToMany.Add(detail);
 				transaction.Commit();
 			}
 		}
@@ -42,6 +44,18 @@ public void LinqFetch()
 			}
 		}
 
+		[Test(Description = "GH-3239")]
+		public void LinqFetchManyToMany()
+		{
+			using (var session = OpenSession())
+			{
+				var result = (from e in session.Query<Document>().Fetch(x => x.RefferedDetailsManyToMany)
+							select e).FirstOrDefault();
+
+				Assert.That(result.RefferedDetailsManyToMany, Has.Count.EqualTo(1));
+			}
+		}
+
 		[Test]
 		public void QueryOverFetch()
 		{
@@ -52,6 +66,16 @@ public void QueryOverFetch()
 			}
 		}
 
+		[Test(Description = "GH-3239")]
+		public void QueryOverFetchManyToMany()
+		{
+			using (var session = OpenSession())
+			{
+				var result = session.QueryOver<Document>().Fetch(SelectMode.Fetch, x => x.RefferedDetailsManyToMany).SingleOrDefault();
+				Assert.That(result.RefferedDetailsManyToMany, Has.Count.EqualTo(1));
+			}
+		}
+
 		[Test]
 		public void LazyLoad()
 		{

src/NHibernate.Test/NHSpecificTest/NH2174/Mappings.hbm.xml

@@ -42,5 +42,18 @@
       </key>
       <one-to-many class="DocumentDetailDocument" />
     </bag>
+    
+    <bag name="RefferedDetailsManyToMany" lazy="true" generic="true" cascade="all">
+      <key>
+        <column name="id_Doc_detail" />
+        <column name="id_base_detail" />
+      </key>
+      <many-to-many class="DocumentDetailDocument" >
+        <column name="id_item" />
+        <column name="Version" />
+        <column name="id_Doc" />
+        <column name="id_base" />
+      </many-to-many>
+    </bag>
   </class>
 </hibernate-mapping>