title: Admin : Select category -> show subcategory using django
body: <p>how to show category and subcategory :
   Admin : Select category -> show subcategory   using Django </p>
  
  <pre><code>class Category(models.Model):
      categoryName = models.CharField(max_length=100)
  
      def __unicode__(self):
          return self.categoryName
  
  class Subcategory(models.Model):
      subcategoryName=models.CharField(max_length=100)
      category = models.ForeignKey(Category, null=True, blank=True)
  
      def __unicode__(self):
          return self.subcategoryName
  
  
  class Product(models.Model):
      productName = models.CharField(max_length=100)
      productImage = models.ImageField(upload_to='documents/')
      category = models.ForeignKey(Category, null=True, blank=True)
      subcategory = models.ForeignKey(Subcategory, null=True, blank=True)
  
      def __unicode__(self):
          return self.productName
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/YenEN.png" rel="nofollow noreferrer">admin product form</a></p>