optional fields in django models -

i have following model in django.

class link(models.model):     name = models.charfield(max_length=100)     url = models.charfield(max_length=100)     tag = models.charfield(max_length=100)      def __unicode__(self):         return self.name 

i need url field optional. how do this?

add attribute blank=true. optionally, can make field nullable null=true.