Foreign Key in Django model

Foreign Key in Django model

Here is my situation. SubCategory has foreign key to topic and Topic has
foreign key to SubCategory.
class SubCategory(models.Model):
name = models.CharField(max_length=100)
slug = models.SlugField(max_length=110)
description = models.TextField(default='')
ordering = models.PositiveIntegerField(default=1)
category = models.ForeignKey(Category)
created_on = models.DateTimeField(auto_now_add=True)
created_by = models.ForeignKey(User)
updated_on = models.DateTimeField(blank=True, null=True)
updated_by = models.ForeignKey(User, related_name='+')
num_topics = models.IntegerField(default=0)
num_posts = models.IntegerField(default=0)
last_topic = models.ForeignKey(Topic, related_name='+')
class Topic(models.Model):
name = models.CharField(max_length=300)
slug = models.SlugField(max_length=300)
description = models.TextField(default='')
subcategory = models.ForeignKey(SubCategory)
created_on = models.DateTimeField(auto_now_add=True)
created_by = models.ForeignKey(User)
updated_on = models.DateTimeField(blank=True, null=True)
updated_by = models.ForeignKey(User, related_name='+')
When I run this code, it gives the following error
NameError: name 'Topic' is not defined.
Can anybody tell me how to fix it ?