4 files changed, 14 insertions, 6 deletions
diff --git a/django/core/serializers/__init__.py b/django/core/serializers/__init__.py
index 75e087ee1b..a1268321f2 100644
--- a/django/core/serializers/__init__.py
+++ b/django/core/serializers/__init__.py
@@ -29,7 +29,7 @@ _serializers = {}
         
 def register_serializer(format, serializer_module):
     """Register a new serializer by passing in a module name."""
-    module = __import__(serializer_module, '', '', [''])
+    module = __import__(serializer_module, {}, {}, [''])
     _serializers[format] = module
     
 def unregister_serializer(format):
diff --git a/django/core/serializers/base.py b/django/core/serializers/base.py
index fb293c7c13..5b0acdc480 100644
--- a/django/core/serializers/base.py
+++ b/django/core/serializers/base.py
@@ -28,6 +28,7 @@ class Serializer(object):
         self.options = options
 
         self.stream = options.get("stream", StringIO())
+        self.selected_fields = options.get("fields")
 
         self.start_serialization()
         for obj in queryset:
@@ -36,11 +37,14 @@ class Serializer(object):
                 if field is obj._meta.pk:
                     continue
                 elif field.rel is None:
-                    self.handle_field(obj, field)
+                    if self.selected_fields is None or field.attname in self.selected_fields:
+                        self.handle_field(obj, field)
                 else:
-                    self.handle_fk_field(obj, field)
+                    if self.selected_fields is None or field.attname[:-3] in self.selected_fields:
+                        self.handle_fk_field(obj, field)
             for field in obj._meta.many_to_many:
-                self.handle_m2m_field(obj, field)
+                if self.selected_fields is None or field.attname in self.selected_fields:
+                    self.handle_m2m_field(obj, field)
             self.end_object(obj)
         self.end_serialization()
         return self.getvalue()
diff --git a/django/core/serializers/python.py b/django/core/serializers/python.py
index 4181bc7f2b..859816c226 100644
--- a/django/core/serializers/python.py
+++ b/django/core/serializers/python.py
@@ -76,7 +76,7 @@ def Deserializer(object_list, **options):
                 m2m_data[field.name] = field.rel.to._default_manager.in_bulk(field_value).values()
                 
             # Handle FK fields
-            elif field.rel and isinstance(field.rel, models.ManyToOneRel):
+            elif field.rel and isinstance(field.rel, models.ManyToOneRel) and field_value is not None:
                 try:
                     data[field.name] = field.rel.to._default_manager.get(pk=field_value)
                 except field.rel.to.DoesNotExist:
diff --git a/django/core/serializers/xml_serializer.py b/django/core/serializers/xml_serializer.py
index 09fff408cf..512b8c6176 100644
--- a/django/core/serializers/xml_serializer.py
+++ b/django/core/serializers/xml_serializer.py
@@ -166,7 +166,11 @@ class Deserializer(base.Deserializer):
         # If it doesn't exist, set the field to None (which might trigger 
         # validation error, but that's expected).
         RelatedModel = self._get_model_from_node(node, "to")
-        return RelatedModel.objects.get(pk=getInnerText(node).strip().encode(self.encoding))
+        # Check if there is a child node named 'None', returning None if so.
+        if len(node.childNodes) == 1 and node.childNodes[0].nodeName == 'None':
+            return None
+        else:
+            return RelatedModel.objects.get(pk=getInnerText(node).strip().encode(self.encoding))
         
     def _handle_m2m_field_node(self, node):
         """