Auto increment a value in Django with respect to the previous one

Question

Is there a way to autoincrement a field with respect to the previous one...e.g if the previous record has the value 09-0001, then the next record should be assigned 09-0002 and so one...ideas? I'm thinking of overriding the save method, but how exactly I'm not so sure

Answer 1

Django won't let you have more than one AutoField in a model, and you already have one for your primary key. So you'll have to override save and will probably need to look back at the table to figure out what to increment.

Something like this:

class Product(models.Model): 
   code = models.IntegerField()
   number = models.IntegerField()
   ...

   def get_serial_number(self): 
      "Get formatted value of serial number"
      return "%.2d-%.3d" % (self.code, self.product)

   def save(self): 
      "Get last value of Code and Number from database, and increment before save"
      top = Product.objects.order_by('-code','-number')[0]
      self.code = top.code + 1
      self.number = top.number + 1
      super(Product, self).save()

   # etc.

Note that without some kind of locking in your save method, you can run into a concurrency problem (two threads trying to store the same values in code and number).

Answer 2

AutoField is the field type that Django uses for the model base class' 'id' property, but you can use it for additional autoincremented fields.

I don't think you can do any formatting like in your example, but a good approach would be to make a custom field type that overrides the save() method to perform the desired formatting.

Answer 3

in SQL:

+-------+------------------+------+-----+---------+----------------+
| Field | Type             | Null | Key | Default | Extra          |
+-------+------------------+------+-----+---------+----------------+
| code  | int(11) unsigned | NO   | PRI | NULL    | auto_increment |
+-------+------------------+------+-----+---------+----------------+

In Django:

self.code = None
self.number = top.number + 1
self.save() # or super(Product, self).save()

When saved, the code will be increased automatically.