def _extract_match_id(match_json):
    """
    Extract the match_id from json response.
    {
        "matches": [
            {
                "id": "0000...",
                "match_id": 1313,
                "similarity": "001000"
            }
        ]
    }
    """
    matches = match_json.get('matches', [])
    if len(matches) == 1:
        return matches[0].get('match_id', None)
    return None