code: public static boolean update(String tableName, String primaryKey, Record record) {
  		return MAIN.update(tableName, primaryKey, record);
  	}
docs: Update Record.
  <pre>
  Example:
  Db.update("user_role", "user_id, role_id", record);
  </pre>
  @param tableName the table name of the Record save to
  @param primaryKey the primary key of the table, composite primary key is separated by comma character: ","
  @param record the Record object
  @param true if update succeed otherwise false