from django.db.models.expressions import F

def __floordiv__(self, other):
    return self._combine(other, '||', False)
F.__floordiv__ = __floordiv__

class CF(F):
    """
    A coalesced expression representing the value of the given field.
    """

    def __init__(self, name, default=''):
        super(CF, self).__init__(name)
        self.default = default

    def evaluate(self, *args, **kwargs):
        res = super(CF, self).evaluate(*args, **kwargs)
        return 'COALESCE(%s, %%s)' % res[0], res[1] + (self.default,)