MappingByCode: Support backfield property access (#3254)

Fixes #3251

Author: bahusoid

src/NHibernate.Test/MappingByCode/ExplicitMappingTests/AllPropertiesRegistrationTests.cs

@@ -188,6 +188,20 @@ public void WhenMapPropertiesInTheBaseJumpedClassUsingMemberNameThenMapInInherit
 			Assert.That(hbmClass.Properties.Select(p => p.Access).All(a => a.StartsWith("field.")), Is.True);
 		}
 
+		[Test]
+		public void BackfieldAccessPropertyMapping()
+		{
+			var mapper = new ModelMapper();
+			mapper.Class<MyClass>(mc =>
+								  {
+									mc.Id(x => x.Id, m => m.Access(Accessor.Backfield));
+								  });
+
+			HbmMapping mappings = mapper.CompileMappingForAllExplicitlyAddedEntities();
+			HbmClass hbmClass = mappings.RootClasses[0];
+			Assert.That(hbmClass.Id.access, Is.EqualTo("backfield"));
+		}
+
 		[Test]
 		public void WhenMapBagWithWrongElementTypeThenThrows()
 		{

src/NHibernate/Mapping/ByCode/IAccessorPropertyMapper.cs

@@ -6,12 +6,13 @@ public enum Accessor
 		Field,
 		NoSetter,
 		ReadOnly,
-		None
+		None,
+		Backfield,
 	}
 
 	public interface IAccessorPropertyMapper
 	{
 		void Access(Accessor accessor);
 		void Access(System.Type accessorType);
 	}
-}
+}

src/NHibernate/Mapping/ByCode/Impl/AccessorPropertyMapper.cs

@@ -92,6 +92,9 @@ public void Access(Accessor accessor)
 				case Accessor.None:
 					setAccessor("none");
 					break;
+				case Accessor.Backfield:
+					setAccessor("backfield");
+					break;
 				default:
 					throw new ArgumentOutOfRangeException("accessor");
 			}
@@ -136,4 +139,4 @@ private static MemberInfo GetField(System.Type type, string fieldName)
 			return member;
 		}
 	}
-}
+}