title: Django Admin - How to pull model default field values from other tables
body: <p>I am trying to get a field value automatically pulled from different table. Below is my case: I have two tables: sales and returns. While entering a new return, when a "sales_id" is selected, want to show "sales_quantity" automatically populated, non-editable (and also if possible, want to constraint the "return_quantity" &lt;= "sales_quantity"). </p>
  
  <pre><code>class T_Sales(models.Model):
      product = models.ForeignKey(P_Product)
      sales_quantity = models.IntegerField()
      def __unicode__(self):             
          return str(self.id)
  
  class T_Return(models.Model):
      sales_id = models.ForeignKey(T_Sales)
      #sales_quantity = models.IntegerField(default=T_Sales.sales_quantity)
      return_quantity = models.IntegerField()
      def __unicode__(self):             
          return self.description
  </code></pre>