Django makemigrations cannot detect change when changing custom field overriding charfield.

[김지환]

# category.py

from django.db.models.fields import CharField


class CategoryField(CharField):
    verbose_name = "category"
    max_length = 40

    NORMAL = 'normal'
    PUBLIC = 'public'

    choices = (
        (NORMAL, "normal"),
        (PUBLIC, "public),
    )

    def __init__(self, *args, **kwargs):
        kwargs['max_length'] = self.max_length
        kwargs['choices'] = self.choices
        kwargs['verbose_name'] = self.verbose_name
        kwargs['blank'] = True
        kwargs['null'] = True
        kwargs['default'] = self.NORMAL
        super().__init__(*args, **kwargs)

# in my models.py

class Car(models.Model):

    category = CategoryField()

When I use custom field overriding django CharField, field changes that `max_length`, `choices` not detected by django makemigrations command.

Add field migration works fine, and verbose_name, blank, null changes are detected by makemigrations command.

However, when I change max_length and choices, django makemigrations cannot detect changes.

I checked return value of my CategoryField's deconstruct method, and the result is fine.

Also, if max_length is set like `CategoryField(max_length=50)`, makemigrations can detect changes.

Why django makemigrations cannot detect changes when set max_length and choices in __init__ method?

[Tim Graham]

It looks like you need to add a CategoryField.deconstruct() method.

https://docs.djangoproject.com/en/stable/howto/custom-model-fields/#custom-field-deconstruct-method

[김지환]

CategoryField subclassed CharField, so I think I don't need to add deconstruct method in CategoryField.

And, when call CategoryField().deconstruct(), the values return correctly.

Why django makemigrations cannot detect max_length and choices change?

2017년 3월 13일 월요일 오후 10시 43분 53초 UTC+9, Tim Graham 님의 말:

[Tim Graham]

CategoryField.__init__() has lines like kwargs['max_length'] = self.max_length

That's going to ignore any custom max_length that you provide in the model field's definition, hence those changes aren't detected in migrations.