Reads approval requests of a given type for a given user.

def ReadApprovalRequests(self,
                           requestor_username,
                           approval_type,
                           subject_id=None,
                           include_expired=False,
                           cursor=None):
    """Reads approval requests of a given type for a given user."""

    query = """
        SELECT
            ar.approval_id,
            UNIX_TIMESTAMP(ar.timestamp),
            ar.approval_request,
            u.username,
            UNIX_TIMESTAMP(ag.timestamp)
        FROM approval_request ar
        LEFT JOIN approval_grant AS ag USING (username_hash, approval_id)
        LEFT JOIN grr_users u ON u.username_hash = ag.grantor_username_hash
        WHERE ar.username_hash = %s AND ar.approval_type = %s
        """

    args = [mysql_utils.Hash(requestor_username), int(approval_type)]

    if subject_id:
      query += " AND ar.subject_id = %s"
      args.append(subject_id)

    query += " ORDER BY ar.approval_id"

    ret = []
    now = rdfvalue.RDFDatetime.Now()
    cursor.execute(query, args)
    for approval_request in _ResponseToApprovalsWithGrants(cursor.fetchall()):
      if include_expired or approval_request.expiration_time >= now:
        ret.append(approval_request)
    return ret