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

def decode(self, json_string):

    default_obj = super(LayerMangler, self).decode(json_string)
    for obj in default_obj:
        obj['pk'] = (obj['pk'] + self.basepk)
        obj['fields']['upload_session'] = obj['pk']
        obj['fields']['service'] = None
        if self.datastore:
            obj['fields']['store'] = self.datastore
        else:
            obj['fields']['store'] = obj['fields']['name']
    return default_obj
