def get_vads_trans_id(vads_site_id, vads_trans_date):
    """
    Returns a default value for vads_trans_id field.

    vads_trans_id field is mandatory. It is composed by 6 numeric
    characters that identifies the transaction. There is a unicity contraint
    between vads_site_id and vads_trans_date (the first 8 characters
    representing the transaction date).

    We consider the probability of having 2 identical
    generated vads_trans_id in the same day as null."""
    vads_trans_id = ""
    for i in range(0, 6):
        vads_trans_id += str(random.randint(0, 9))
    return vads_trans_id