CODEWITHSUNDEEP
pythondjangodjango-modelsdjango-admin
Sirion
Sirion

Reputation: 929

Set default value of a field using classmethod in Django

I've found many similar posts on the web about this topic but no one state clearly which is the problem.

Code

class Item(models.Model):
    @classmethod
    def get_next_item_number(cls):
        return cls.objects.count() + 1

    number = models.IntegerField(default=get_next_item_number)

Problem

When I access Django admin panel the text field related to 'number' field contains

<classmethod object at 0x7fab049d7c50>

So I tried to change the code above 

class Item(models.Model):
    @classmethod
    def get_next_item_number(cls):
        return cls.objects.count() + 1

    number = models.IntegerField(default=get_next_item_number())

but when I run django server I get:

number = models.IntegerField(default=get_next_item_number())
TypeError: 'classmethod' object is not callable

I know all this could be prevented by declaring get_next_item_number() as an external function but this solution is not elegant to me because get_next_item_number() refer only to Item class.

Is there a solution I'm missing?

Upvotes: 8

Views: 3171

Answers (2)

Sirion
Sirion

Reputation: 929

I found this solution:

Code

class Item(models.Model):
    @staticmethod
    def get_next_item_number():
        return Item.objects.count() + 1

    number = models.IntegerField(default=get_next_item_number.__func__)

I'm not completely aware of the possible consequences, but it works.

Upvotes: 6

xecgr
xecgr

Reputation: 5193

Overwrite __init__ method of your Item class and assing the default value there, have a look to this code:

from django.db import models
class Item(models.Model):
    number = models.IntegerField()
    @classmethod
    def get_next_item_number(cls):
        return cls.objects.count() + 1
    def __init__(self, *args, **kwargs):
        super(Item, self).__init__(*args, **kwargs)
        if not self.pk and not self.number:
            self.number = Item.get_next_item_number()

And here's the flow output

>>> from your_app import models
>>> models.Item().save()
>>> models.Item().number
>>> 2
>>> models.Item().save()
>>> models.Item().number
>>> 3
>>> models.Item.objects.filter(id=1)[0].number #number only get its value from classmethod on new items
>>> 1

Upvotes: 0

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