Store ordered list of field names, and return the ordered list when iterating a document instance.

mongoengine/base.py

@@ -560,8 +560,11 @@ class DocumentMetaclass(type):
 
         # Set _fields and db_field maps
         attrs['_fields'] = doc_fields
-        attrs['_db_field_map'] = dict([(k, getattr(v, 'db_field', k))
-                                        for k, v in doc_fields.iteritems()])
+        attrs['_fields_ordered'] = tuple(i[1]
+                                         for i in sorted((v.creation_counter, v.name)
+                                                          for v in doc_fields.itervalues()))
+        attrs['_db_field_map'] = dict((k, getattr(v, 'db_field', k))
+                                       for k, v in doc_fields.iteritems())
         attrs['_reverse_db_field_map'] = dict(
                 (v, k) for k, v in attrs['_db_field_map'].iteritems())
 
@@ -1302,7 +1305,10 @@ class BaseDocument(object):
         return value
 
     def __iter__(self):
-        return iter(self._fields)
+        if 'id' in self._fields and 'id' not in self._fields_ordered:
+            return iter(('id', ) + self._fields_ordered)
+        
+        return iter(self._fields_ordered)
 
     def __getitem__(self, name):
         """Dictionary-style field access, return a field's value if present.