Django: Generate a custom series number upon saving the model

Question

I am new to django.

I have an Item model which has a property_number field and a FK location field to Location model. property_number has a format like this 0000-00-00-0000-00 which is [year] - [sub account] - [gl account] - [series number] - [location code]. Under the format of series_number, this should be generate upon saving base on the parameter location_code.

Exmaple:

Location code: 10

Item 1

property number should be: 2021 - 01 - 01 - 0001 - 10

Item 2

property number should be: 2021 - 01 - 01 - 0002 - 10

Item 3

property number should be: 2021 - 01 - 01 - 0003 - 10 ... and so on,

property number should be: 2021 - 01 - 01 - 1000 - 10

Location code: 50

Item 1

property number should be: 2021 - 01 - 01 - 0001 - 50

Item 2

property number should be: 2021 - 01 - 01 - 0002 - 50 ... and so on,

property number should be: 2021 - 01 - 01 - 0055 - 50

Location code: 65

Item 1

property number should be: 2021 - 01 - 01 - 0001 - 65 ... and so on,

property number should be: 2021 - 01 - 01 - 0100 - 65

*In this example, for the location code 10, item 1, item 2 and item 3 has a series number 0001, 0002 and 0003

and for the location code 50, item 1 and item 2 has number series of 0001 and 0002

and so forth

Each time a new Item will be saved with different location code, the number_series will be reset to 0001, and will be continued when the new Item is save with the same location code

What I have so far

I can already save the format by overriding the save function, but I have no idea on how to generate the desire series number.

def save(self, *args, **kwargs):
    self.property_number = '{}-{}-{}-{}'.format(self.date_acquired.year, self.sub_major_group_and_gl_account.sub_major_group_and_gl_account, 'This is where the series_number is place', self.location.location_code)
    super(Item, self).save(*args, **kwargs)

This is what I have so far to generate the series_number, but of course this is not correct. I also don't know how to do comparison of the new data to be save and the already saved data.

def generate_series_number(self):
    # Get all records and filter by location.location_code and should be compare if there is already a data that has the same location_code
    get_item_location = Item.objects.all().filter(self.location.location_code=='This should be the latest data to be save')

    # Count how many data has the same location_code to know what number series to be assigned
    # if result is, there are already 50 records exist with location_code = '01', the new data will have the series number '51' 
    count_current_data = get_item_location.count()
    assign_series_number = count_current_data + 1

    return assign_series_number

# I will use the 'generate_series_number' function in the save function to format my Property_number field in the model.
def save(self, *args, **kwargs):
    self.property_number = '{}-{}-{}-{}'.format(self.date_acquired.year, self.sub_major_group_and_gl_account.sub_major_group_and_gl_account, 'series_number', self.location.location_code)
    super(Item, self).save(*args, **kwargs)

This is my Item model

class Item(models.Model):
    item                    = models.CharField(max_length=100, null=True)
    description             = models.TextField(max_length=200, null=True)
    date_acquired           = models.DateField(null=True)
    old_property_number     = models.CharField(max_length=100, null=True)
    property_number         = models.CharField(max_length=100, null=True, blank=True)
    unit_of_measure         = models.ForeignKey('Uom', related_name='uom_item', null=True, on_delete=models.SET_NULL)
    unit_value              = models.DecimalField(max_digits=12, decimal_places=2, null=True)
    quantity_per_card           = models.CharField(max_length=100, null=True, default='1')
    quantity_per_physical_count = models.CharField(max_length=100, null=True, default='1' )
    # location_code as primary key
    location                    = models.ForeignKey('Location', related_name='location_item', null=True, on_delete=models.SET_NULL)
    condition                   = models.ForeignKey('Condition', related_name='condition_item', null=True, on_delete=models.SET_NULL)
    accountable_person          = models.ForeignKey('Personnel', related_name='personnel_item', null=True, on_delete=models.SET_NULL)
    remarks                     = models.ForeignKey('Remarks', related_name='remarks_item', null=True, on_delete=models.SET_NULL)
    sub_major_group_and_gl_account    = models.ForeignKey('Account', related_name='sub_major_group_and_gl_account_item', null=True, on_delete=models.SET_NULL)

NOTE: I don't have a view.py, what I am working is within the admin site only.

I am new to Django, don't be mad at me :) Thanks in advance.

Answer 1

I am answering my own question.

I managed to solved my issues with the below save function. if anyone can improve my solution, it will be appreciated.

Note: This is only applicable to newly added instance via admin site and WILL NOT WORK if the instance are imported via django-import-export. So, that would be the remaining issue for me, to automatically assigned the data_counter for every instance when using the import function in admin site.

Else, will do the assignment manually in the excel file. XD

def save(self, *args, **kwargs):
    data_counter = Item.objects.filter(location=self.location).aggregate(counter=Count('id'))['counter'] # Count existing instance with matching "location id" of the new instance
    if self.pk is None and self.property_number is None: # if new instance and property number field is blank
        if data_counter is None: # if data_counter is empty/none, no matching instance
            data_counter = 1  # start at 1
        else: # else, if there is matching instance
            data_counter += 1 # add 1 to the data_counter
    elif self.pk is not None and self.property_number is None: # if not new instance and property number field is blank, Update an instance without property number
        if data_counter is None: # if data_counter is empty/none, the existing instance has no matching location id, Update
            data_counter = 1 # Existing instance start at 1
        else: # else, if there is matching instance
            data_counter += 1 # add 1 to the data_counter

    # data_counter will be the series number for every location ID
    self.property_number = '{}-{}-{:04d}-{}'.format(self.date_acquired.year, self.sub_major_group_and_gl_account.sub_major_group_and_gl_account, data_counter, self.location.location_code)
    
    super(Item, self).save(*args, **kwargs)