Split a field from _sqlFields into table, column.

        Registers the table in cls._tables, and returns a fully
        qualified table.column (default table: cls._sqlTable)

def _checkTable(cls, field):
        """Split a field from _sqlFields into table, column.

        Registers the table in cls._tables, and returns a fully
        qualified table.column (default table: cls._sqlTable)
        """
        # Get table part
        try:
            (table, field) = field.split('.')
        except ValueError:
            table = cls._sqlTable
        # clean away white space
        table = table.strip()
        field = field.strip()
        # register table
        cls._tables[table] = None
        # and return in proper shape
        return table + '.' + field