added OrderedDocument class to decode BSON data to OrderedDict for retrieving data in order

mongoengine/document.py

@@ -1,6 +1,7 @@
 import re
 import warnings
 
+from collections import OrderedDict
 from bson.dbref import DBRef
 import pymongo
 from pymongo.read_preferences import ReadPreference
@@ -21,7 +22,8 @@ from mongoengine.queryset import (NotUniqueError, OperationError,
 
 __all__ = ('Document', 'EmbeddedDocument', 'DynamicDocument',
            'DynamicEmbeddedDocument', 'OperationError',
-           'InvalidCollectionError', 'NotUniqueError', 'MapReduceDocument')
+           'InvalidCollectionError', 'NotUniqueError', 'MapReduceDocument',
+           'OrderedDocument')
 
 
 def includes_cls(fields):
@@ -1036,3 +1038,27 @@ class MapReduceDocument(object):
             self._key_object = self._document.objects.with_id(self.key)
             return self._key_object
         return self._key_object
+
+
+class OrderedDocument(Document):
+    """A document that is almost same with Document except for returning
+    results in OrderedDict instead of dict.
+    """
+
+    # The __metaclass__ attribute is removed by 2to3 when running with Python3
+    # my_metaclass is defined so that metaclass can be queried in Python 2 & 3
+    my_metaclass = TopLevelDocumentMetaclass
+    __metaclass__ = TopLevelDocumentMetaclass
+
+    @classmethod
+    def _get_collection(cls):
+        collection = super(OrderedDocument, cls)._get_collection()
+
+        if IS_PYMONGO_3:
+            # returns collection object which is set OrderedDict class to be decoded from BSON document
+            from bson import CodecOptions
+            return collection.with_options(codec_options=CodecOptions(document_class=OrderedDict))
+        else:
+            # set attribute to specify the class to be decoeded
+            cls.decoded_class = OrderedDict
+            return collection

mongoengine/queryset/base.py

@@ -1501,6 +1501,10 @@ class BaseQuerySet(object):
                 cursor_args['read_preference'] = self._read_preference
             else:
                 cursor_args['slave_okay'] = self._slave_okay
+
+            # set decode format if needed
+            if hasattr(self._document, 'decoded_class'):
+                cursor_args['as_class'] = self._document.decoded_class
         else:
             fields_name = 'projection'
             # snapshot is not handled at all by PyMongo 3+