'Object-level validation - implemented in this method so DRF serializers
catch errors in advance of a save() attempt.'

def clean(self):

    if (self.is_professional_slug(self.mode_slug) and (self.expiration_datetime is not None)):
        raise ValidationError(_(u'Professional    education    modes    are    not    allowed    to    have    expiration_datetime    set.'))
    if (self.is_verified_slug(self.mode_slug) and (self.min_price <= 0)):
        raise ValidationError(_(u'Verified    modes    cannot    be    free.'))
