Added creation_counter to BaseField in order to provied form modules with a way to sort fields i order user specified them (same technique is used in Django)

mongoengine/base.py

@@ -25,6 +25,12 @@ class BaseField(object):
     _index_with_types = True
     _geo_index = False
 
+    # These track each time a Field instance is created. Used to retain order.
+    # The auto_creation_counter is used for fields that MongoEngine implicitly
+    # creates, creation_counter is used for all user-specified fields.
+    creation_counter = 0
+    auto_creation_counter = -1
+
     def __init__(self, db_field=None, name=None, required=False, default=None, 
                  unique=False, unique_with=None, primary_key=False,
                  validation=None, choices=None):
@@ -41,6 +47,13 @@ class BaseField(object):
         self.primary_key = primary_key
         self.validation = validation
         self.choices = choices
+        # Adjust the appropriate creation counter, and save our local copy.
+        if self.db_field == '_id':
+            self.creation_counter = BaseField.auto_creation_counter
+            BaseField.auto_creation_counter -= 1
+        else:
+            self.creation_counter = BaseField.creation_counter
+            BaseField.creation_counter += 1
 
     def __get__(self, instance, owner):
         """Descriptor for retrieving a value from a field in a document. Do