To achive storing object data in order with minimum implementation, I

changed followings.

- added optional parameter `container_class` which enables to choose
  intermediate class at encoding Python data, instead of additional
  field class.
- removed OrderedDocument class because the equivalent feature could
  be implemented by the outside of Mongoengine.

mongoengine/fields.py

@@ -5,7 +5,6 @@ import re
 import time
 import uuid
 import warnings
-from collections import OrderedDict
 from operator import itemgetter
 
 from bson import Binary, DBRef, ObjectId, SON
@@ -50,7 +49,7 @@ __all__ = (
     'FileField', 'ImageGridFsProxy', 'ImproperlyConfigured', 'ImageField',
     'GeoPointField', 'PointField', 'LineStringField', 'PolygonField',
     'SequenceField', 'UUIDField', 'MultiPointField', 'MultiLineStringField',
-    'MultiPolygonField', 'GeoJsonBaseField', 'OrderedDynamicField'
+    'MultiPolygonField', 'GeoJsonBaseField'
 )
 
 RECURSIVE_REFERENCE_CONSTANT = 'self'
@@ -620,6 +619,14 @@ class DynamicField(BaseField):
 
     Used by :class:`~mongoengine.DynamicDocument` to handle dynamic data"""
 
+    def __init__(self, container_class=dict, *args, **kwargs):
+        self._container_cls = container_class
+        if not issubclass(self._container_cls, dict):
+            self.error('The class that is specified in `container_class` parameter '
+                       'must be a subclass of `dict`.')
+
+        super(DynamicField, self).__init__(*args, **kwargs)
+
     def to_mongo(self, value, use_db_field=True, fields=None):
         """Convert a Python type to a MongoDB compatible type.
         """
@@ -645,7 +652,7 @@ class DynamicField(BaseField):
             is_list = True
             value = {k: v for k, v in enumerate(value)}
 
-        data = self._container_type() if hasattr(self, '_container_type') else {}
+        data = self._container_cls()
         for k, v in value.iteritems():
             data[k] = self.to_mongo(v, use_db_field, fields)
 
@@ -676,16 +683,6 @@ class DynamicField(BaseField):
             value.validate(clean=clean)
 
 
-class OrderedDynamicField(DynamicField):
-    """A field that wraps DynamicField. This uses OrderedDict class
-    to guarantee to store data in the defined order instead of dict.
-    """
-
-    def __init__(self, *args, **kwargs):
-        super(OrderedDynamicField, self).__init__(*args, **kwargs)
-        self._container_type = OrderedDict
-
-
 class ListField(ComplexBaseField):
     """A list field that wraps a standard field, allowing multiple instances
     of the field to be used as a list in the database.