Return a list of fields with a prop key defined

        Each list item will be a tuple of field name containing
        the prop key & the value of that prop key.

        :param prop_key: key name
        :return: list of tuples

def get_fields_with_prop(cls, prop_key):
        """ Return a list of fields with a prop key defined

        Each list item will be a tuple of field name containing
        the prop key & the value of that prop key.

        :param prop_key: key name
        :return: list of tuples
        """

        ret = []

        for key, val in getattr(cls, '_fields').items():
            if hasattr(val, prop_key):
                ret.append((key, getattr(val, prop_key)))
        return ret