filtering dropdown values in django admin

class Foo(models.Model):
    title = models.TextField()
    userid = models.IntegerField()
    image = models.CharField(max_length=100)
    def __unicode__(self):
       return self.title

class Bar(models.Model):
    foo = models.ForeignKey(Foo, related_name='Foo_picks', unique=True)
    added_on = models.DateTimeField(auto_now_add=True)

In Django admin add_view:

def add_view(self, *args, **kwargs):
    self.exclude = ("added_on",)
    self.readonly_fields = ()
    return super(Bar, self).add_view(*args, **kwargs)

So, Field shows in the admin add view is foo Which is a drop down list and shows all the titles. Some title of Foo remains empty or ''. So, drop down list have lots of empty value because it title is empty. I want to filter out those empty values.