[FIXED] Django auto increment fields

Issue

I have 2 columns named Serial and Bag I need them to be auto incremented but based on each other and also based on the user that will update the record, so every bag should have 100 serial and reset the number automatically after reaching 100, then start again with Bag number 2 and put 100 serial in it and reset.

For example:

when user update the first record the Bag will start with number 1 and Serial will be also number 1 the second record Bag will still number 1 and the serial will be changed to number 2 till reach 100 Serial in one Bag, then we will start again with bag number 2 and serial number 1 etc ...

Thanks

Solution

The way you explain your example is a bit confusing but I'll try to give you an answer.

I assume the "2 columns named Serial and Bag" are fields of the same model and as you replied in the comments "the record is already existing but it has empty serial and bag", which means the auto-increment begins when the record is updated. Lastly, you mentioned first and second records implying that there are multiple records in this model. Based on these criteria, what you can do is add a save method in your model:

# Sample model
class Record(models.Model):
  bag = models.IntegerField(default=0, null=True)
  serial = models.IntegerField(default=0, null=True)
  created_at = models.DateTimeField(auto_now=True, null=True)

  def save(self, *args, **kwargs):
    # Ensures the record will only auto-increment during update
    if self.created_at:
      # Retrieves the object with the highest bag & serial value
      latest_record = Record.objects.all().order_by('bag', 'serial').last()

      # Incrementing logic
      if latest_record.serial_no + 1 <= 100:
        self.bag = latest_record.bag if latest_record.bag > 0 else 1
        self.serial = latest_record.serial + 1
      else:
        self.bag = latest_record.bag + 1
        self.serial = 1

    super(Record, self).save(*args, **kwargs)

Now, each time you write save like:

record = Record()
record.save()

The model save method executes.

Answered By - Christopher Tabula