added queryset chaining via 'filter' method. test included.

mongoengine/base.py

@@ -19,6 +19,7 @@ class BaseField(object):
         self.default = default
         self.unique = bool(unique or unique_with)
         self.unique_with = unique_with
+        self._loaded = []
 
     def __get__(self, instance, owner):
         """Descriptor for retrieving a value from a field in a document. Do 
@@ -68,7 +69,7 @@ class ObjectIdField(BaseField):
     """
     
     def to_python(self, value):
-        return str(value)
+        return unicode(value)
 
     def to_mongo(self, value):
         if not isinstance(value, pymongo.objectid.ObjectId):

mongoengine/queryset.py

@@ -62,6 +62,9 @@ class QuerySet(object):
         query = QuerySet._transform_query(_doc_cls=self._document, **query)
         self._query.update(query)
         return self
+        
+    def filter(self, **query):
+        return self.__call__(**query)
 
     @property
     def _collection(self):