Allow to use form_extra_fields

wtforms_appengine/ndb.py

@@ -412,7 +412,7 @@ def model_fields(model, only=None, exclude=None, field_args=None,
 
 
 def model_form(model, base_class=Form, only=None, exclude=None,
-               field_args=None, converter=None):
+               field_args=None, converter=None, extra_fields=None):
     """
     Creates and returns a dynamic ``wtforms.Form`` class for a given
     ``ndb.Model`` class. The form class can be used as it is or serve as a base
@@ -439,6 +439,9 @@ def model_form(model, base_class=Form, only=None, exclude=None,
     # Extract the fields from the model.
     field_dict = model_fields(model, only, exclude, field_args, converter)
 
+    if extra_fields:
+        field_dict.update(extra_fields)
+
     # Return a dynamically created form class, extending from base_class and
     # including the created fields as properties.
     return type(model._get_kind() + 'Form', (base_class,), field_dict)