Django model max_length from another Django model

Question

I have two models. The model Copy should get the max_length from the model AdSpot without inheriting from it. And after trying multiple times, I failed to make this work:

class AdSpot(models.Model):

    title_max_chars = models.IntegerField(default=0)

    @property
    def title_max_chars_from_adspot(self):
        return self.title_max_chars

class Copy(models.Model):
    adspot = models.ForeignKey(AdSpot, on_delete=models.PROTECT)
    def title_max_chars_from_adspot(self, *args, **kwargs):
        return self.adspot.title_max_chars_from_adspot
    title = models.CharField('Name', max_length=title_max_chars_from_adspot, default="")

The error is

polls.Copy.title: (fields.E121) 'max_length' must be a positive integer.

What am I missing ?

TypeError: int() argument must be a string, a bytes-like object or a number, not 'function' — 4m1nh4j1
– 4m1nh4j1, Commented Jun 7, 2019 at 16:11
Which behavior do you expect? Each row in Copy model must have different column title width depending on linked AdSpot row? — IVNSTN
– IVNSTN, Commented Jun 7, 2019 at 16:15
You can't use a computed value there because that is a database constraint, and it needs to exists when you apply the migration. You will need to validated that on your save method. — Stargazer
– Stargazer, Commented Jun 7, 2019 at 16:16

gachdavit · Accepted Answer · 2020-01-29 09:44:14Z

2

You are doing things in a wrong way. max_length should be integer, and your are passing python's FunctionType object. title_max_chars_from_adspot is a property of Copy class and you do not have instance of Copy class (You will never get it during makemigration/migration) process. The only way is to pass int max_length=5 for example, or write @staticmethod @classmethod for Copy class (But this case, it will be impossible to get access on ForeignKey object). Change strategy.

class AdSpot(models.Model):

    title_max_chars = models.IntegerField(default=22)

class Copy(models.Model):
    adspot = models.ForeignKey(AdSpot, on_delete=models.PROTECT)

    default_length = 255
    for field in AdSpot._meta.fields:
        if field.name == 'title_max_chars':
            default_length = field.default
    title = models.CharField('Name', max_length=default_length, default='') # default value of title_max_chars (22 this case) from AdSpot will be set as a max_length of title

edited Jan 29, 2020 at 9:44

answered Jun 7, 2019 at 16:14

gachdavit

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1 Comment

4m1nh4j1 Over a year ago

thanks, I did some initial test and this is working

Erfan · Accepted Answer · 2019-06-07 16:15:41Z

0

You are setting Adspot.title_max_chars to a default equals to 0 However max_length always should be a positive integer. try not to provide a default.

answered Jun 7, 2019 at 16:15

Erfan

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Comments

khelwood · Accepted Answer · 2019-06-08 06:45:50Z

0

You can use PositiveIntegerField in place of IntegerField.

class AdSpot(models.Model):

    title_max_chars = models.PositiveIntegerField(default=0)

    @property
    def title_max_chars_from_adspot(self):
        return self.title_max_chars

edited Jun 8, 2019 at 6:45

khelwood

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answered Jun 7, 2019 at 16:37

DEEPAK KUMAR

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1 Comment

4m1nh4j1 Over a year ago

Copy.title: (fields.E121) 'max_length' must be a positive integer.

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