'Handle deletions of fields'

def __delattr__(self, *args, **kwargs):

    field_name = args[0]
    if (field_name in self._fields):
        default = self._fields[field_name].default
        if callable(default):
            default = default()
        setattr(self, field_name, default)
    else:
        super(BaseDocument, self).__delattr__(*args, **kwargs)
