title: Django - Positive/Negative Integerfield depending on choice
body: <p>I have a model which consists of cashflows: positive and negative ones. I would like to enforce a positive or negative amount when a user fills in a form.</p>
  
  <p>So basically:</p>
  
  <blockquote>
    <ol>
    <li>first you choose inflow or outflow</li>
    <li>next you type in the amount. the sign will depend on your previous choice</li>
    </ol>
  </blockquote>
  
  <p>How can I achieve this? I am aware of PositiveIntegerField which works such as Integerfield but don't see how to integrate this in the model?</p>
  
  <pre><code>class CashFlow(models.Model):
  
   POSITIVE= 'inflow'
   NEGATIVE= 'outflow'
  
  cashflowchoices = (
      (POSITIVE, 'inflow'),
      (NEGATIVE, 'outflow'),
      )
  
  type = models.CharField(max_length=20, choices = cashflowchoices, default=CALL)
  amount = models.DecimalField(max_digits=20, decimal_places=2, default='1')
  </code></pre>