"""

Useful auxilliary data structures for query construction. Not useful outside

the SQL domain.

"""

class EmptyResultSet(Exception):

    pass

class FullResultSet(Exception):

    pass

class MultiJoin(Exception):

    """

    Used by join construction code to indicate the point at which a

    multi-valued join was attempted (if the caller wants to treat that

    exceptionally).

    """

    def __init__(self, level):

        self.level = level

class Empty(object):

    pass

class RawValue(object):

    def __init__(self, value):

        self.value = value

class Date(object):

    """

    Add a date selection column.

    """

    def __init__(self, col, lookup_type):

        self.col = col

        self.lookup_type = lookup_type

    def relabel_aliases(self, change_map):

        c = self.col

        if isinstance(c, (list, tuple)):

            self.col = (change_map.get(c[0], c[0]), c[1])

    def as_sql(self, qn, connection):

        if isinstance(self.col, (list, tuple)):

            col = '%s.%s' % tuple([qn(c) for c in self.col])

        else:

            col = self.col

        return connection.ops.date_trunc_sql(self.lookup_type, col)