title: Django - default field value depends on other field value
body: <p><br>I have a problem to set a default field value. What I want to do?<br>I want that price in class Packages be a default value of priceNoTax in class Bill. As you can see, all three classes are 
  logically connected.<br>Example: Account 1 has a package with id 1. Price of this package is 100. Default value of priceNoTax for Account 1 is 100.<br><br>How to do that? I am relative new at this, so I need help.<br></p>
  
  <p>models.py</p>
  
  <pre><code>class Packages(models.Model):
       #other fields
       price = models.IntegerField(validators=[MinValueValidator(1)], verbose_name="Price of package")
  
  class Account(models.Model):
       startDate = models.DateField(verbose_name="Start date")
       finishDate = models.DateField(verbose_name="Finish date")
       idPackage = models.ForeignKey(Packages, on_delete=models.CASCADE, verbose_name="Package")
  
  class Bill(models.Model):
       date = models.DateField(default=datetime.now())
       tax = models.FloatField(default=0.20)
       priceNoTax = models.IntegerField()
       priceTax = models.FloatField(default=priceNoTax+(priceNoTax*tax))
       idAccount = models.ForeignKey(Account, on_delete=models.CASCADE, verbose_name="Account")
  
       def __str__(self):
           return self.date
  </code></pre>
  
  <p>Thanks a lot!!!</p>