'json_string is basicly string that you give to json.loads method'

def decode(self, json_string):

    default_obj = super(LayerAttributesMangler, self).decode(json_string)
    for obj in default_obj:
        obj['pk'] = (obj['pk'] + self.basepk)
        obj['fields']['layer'] = (obj['fields']['layer'] + self.basepk)
    return default_obj
