title: NameError: name 'Area' is not defined
body: <p>I made area data model in models.py.</p>
  
  <pre><code>#coding:utf-8
  from django.db import models
  class User(models.Model):
      name = models.CharField(max_length=200,null=True)
      age = models.CharField(max_length=200,null=True)
      area = models.ForeignKey(Area)
  
  class Area(models.Model):
      name = models.CharField(max_length=20, verbose_name='area')
  
  class Prefecture(models.Model):
      name = models.CharField(max_length=20, verbose_name='city')
      area = models.ForeignKey(Area)
  
  class City(models.Model):
      name = models.CharField(max_length=20, verbose_name='region')
              prefecture = models.ForeignKey(Prefecture)
  
  class Price(models.Model):
      name = models.CharField(max_length=20, verbose_name='price')
              PRICE_RANGE = (
                  ('a', 'under500'),
                  ('b', '500-1000'),
                  ('c', 'upper1000'),
              )
      price_range = models.CharField(max_length=1, choices=PRICE_RANGE)
      city = models.ForeignKey(City)
  </code></pre>
  
  <p>When I run migration command,<code>NameError: name 'Area' is not defined area = models.ForeignKey(Area)</code> error happens.However,I defined Area,so I really cannot understand why this error happens.How should I fix this?</p>