Process the input file and fill the DB.

def migrate(self):
        """Process the input file and fill the DB."""
        with io.open(self.json_file, 'rt', encoding='utf8') as fh:
            threads = json.load(fh)
        progress = Progress(len(threads))

        for i, thread in enumerate(threads):
            progress.update(i, str(i))
            self.insert(thread)

        progress.finish("{0} threads, {1} comments".format(len(threads), self.count))