data = {}
for field in self._fields.keys():
  if field in form and form[field].data != getattr(self, field):
    if field in request.files:
      getattr(self, field).put(request.files[field].stream)
    elif field == "themes":
      data[field] = [Theme(name=form[field].data)]
    else:
     data[field] = form[field].data

self.update(**data)