Django Models: Generate field content from other fields in model instance

Question

As an example, say I'm logging inventory items, and I need a unique identifier (that is also meaningful when read by itself) for each item.

The following shows the model I am trying

class InventoryItem(models.Model):
    item_name = models.TextField(max_length = 100)
    item_manufacturer = models.TextField(max_length = 20)
    item_product_num = models.TextField(max_length = 25)
    item_lot = models.TextField(max_length = 25)
    item_received_on = models.DateTimeField(auto_now_add = True)
    item_price = models.DecimalField()
    item_quantity = models.DecimalField()
    item_special_instructions = models.TextField(default = "NA", max_length = 200)
    item_reinspection_date = models.DateField()

    def makeUniqueID(self):
        unique_str = self.item_manufacturer + self.item_product_num + self.item_lot + datetime.datetime.now().strftime("%Y%m%d%H%M%S")
        return unique_str
    item_uniqueID = models.TextField(max_length = 50, default = self.makeUniqueID())

The above code errors NameError: name 'self' is not defined , and I suspect this is not the correct way to do this. Any help is greatly appreciated!

The database I'm using is SQLite if that changes anything

Answer 1

I would do this by overriding the save function on the model.

class InventoryItem(models.Model):
    #Other Fields
    item_uniqueID = models.TextField(max_length = 50)
    def save(self, *args, **kwargs):
        self.item_uniqueID = self.item_manufacturer + self.item_product_num + self.item_lot + datetime.datetime.now().strftime("%Y%m%d%H%M%S")
        super(InventoryItem, self).save(*args, **kwargs)

Answer 2

Possibly try the following:

class InventoryItem(models.Model):
    item_name = models.TextField(max_length = 100)
    item_manufacturer = models.TextField(max_length = 20)
    item_product_num = models.TextField(max_length = 25)
    item_lot = models.TextField(max_length = 25)
    item_received_on = models.DateTimeField(auto_now_add = True)
    item_price = models.DecimalField()
    item_quantity = models.DecimalField()
    item_special_instructions = models.TextField(default = "NA", max_length = 200)
    item_reinspection_date = models.DateField()
    item_uniqueID = models.TextField(max_length = 50)

    def save(self, *args, **kwargs):
        if not self.item_uniqueID:
            self.item_uniqueID = (self.item_manufacturer +
                                  self.item_product_num +
                                  self.item_lot +
                                  datetime.datetime.now().strftime("%Y%m%d%H%M%S"))
            super(InventoryItem, self).save(*args, **kwargs)