Default value for field in Django model

Question

Suppose I have a model:

class SomeModel(models.Model):
   id = models.AutoField(primary_key=True)
   a = models.IntegerField(max_length=10)
   b = models.CharField(max_length=7)

Currently I am using the default admin to create/edit objects of this type. How do I set the field 'a' to have the same number as id? (default=???)

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Other question

Suppose I have a model:

   event_date = models.DateTimeField( null=True)
      year = models.IntegerField( null=True)
      month = models.CharField(max_length=50, null=True)
      day = models.IntegerField( null=True)

How can i set the year, month and day fields by default to be the same as event_date field?

Answer 1

You can override the save() method, and if a is still empty/null at that point, copy the id field. Same goes for your 2nd question.

Answer 2

does it HAVE to be a field stored in the database? (redundancy is bad yaddayadda)

If you just want to call it from inside python, use this:

event_date = models.DateTimeField( null=True)

@property
def year(self):
    return self.event_date and self.event_date.strftime("%Y") or None