Can't pickle Transaction objects about chainermn HOT 2 CLOSED

class lsun_bedroom_train(datasets_base):
    def _init_db(self):    # <-------------
        self.db = lmdb.open(path+"/bedroom_train_lmdb", readonly=True).begin(write=False)

    def __init__(self, path, img_size=256):
        self.all_keys = self.read_image_key_file_json(path + '/key_bedroom.json')
        self.db = None
        super(lsun_bedroom_train, self).__init__(flip=1, resize_to=img_size, crop_to=0)

    def __len__(self):
        return len(self.all_keys)

    def get_example(self, i):
        if self.db is None:    # <-------------
            self._init_db()
        id = self.all_keys[i]
        img = None
        val = self.db.get(id.encode())
        img = cv2.imdecode(np.fromstring(val, dtype=np.uint8), 1)
        img = self.do_augmentation(img)
        img = self.preprocess_image(img)
        return img