About Django's deconstruct and deconstructible

Overview

--Django has a built-in migration feature since 1.7. -In order to automatically extract differences with ** makemigrations **, it is necessary to work on custom fields and custom validators of Model. --Field requires implementation of ** deconstruct ** method --Validator requires ** deconstructible ** decorator and ** \ _ \ _ eq \ _ \ _ ** method implementation ――Easy to summarize the correspondence contents.

Error encountered

--When I created a custom validator and ran ** makemigrations **, I ran into the following error:

python


There are some values Django cannot serialize into migration files.
For more, see https://docs.djangoproject.com/en/1.9/topics/migrations/#migration-serializing

--When I tried to solve this, I came to the story of ** deconstruct **.

Custom validator

--Class-based custom validators were written as follows before 1.6

# validators.py ----

from django.core.exceptions import ValidationError

class UploadSizeValidator:

    def __init__(self, size=None):
        if size:
            self.size = size

    def __call__(self, value):
        if value.file.size > self.size:
            raise ValidationError('Upload Size Error')

# models.py ----

class Question(models.Model):

    question_image = models.ImageField(
        upload_to='question_image',
        validators=[UploadSizeValidator(size=200)],
        default=None
    )

--However, from 1.7, the above error is displayed unless Validator is implemented as follows.

from django.core.exceptions import ValidationError
from django.utils.deconstruct import deconstructible

@deconstructible # <-Add decorator
class UploadSizeValidator:

    def __init__(self, size=None):
        if size:
            self.size = size

    def __call__(self, value):
        if value.file.size > self.size:
            raise ValidationError('Upload Size Error')

    def __eq__(self, other): # <- __eq__Implement method
        return (
            isinstance(other, self.__class__) and (self.size == other.size)
        )

--This is the first time that ** make migrations ** passes. -The reason why ** \ _ \ _ eq \ _ \ _ ** is necessary is that it is used for difference comparison when generating a migration file. ――Specifically, the following things happen.

1. UploadSizeValidator(size=100)Suppose you created a migration file with
2.Change size to 200 and run makemigrations
3. __eq__Before change in method(other.size=100)And after the change(self.size=200)Compare size
4.AlterField migration files are generated because they do not match

――The content of the actually generated migration looks like this

operations = [
    migrations.AlterField(
        model_name='question',
        name='question_image',
        field=models.ImageField(
          default=None, upload_to='question_image',
          validators=[polls.validators.UploadSizeValidator(size=200)]
        ),
    ),

--By doing this, the contents of the validator can also be migrated or returned with backwards. ――By the way, function-based validators do not have anything that can be compared by difference, so it is the same as the writing method so far. --AlterField is only generated when specified as validators

def validate_even(value):
    if value % 2 != 0:
        raise ValidationError(
            _('%(value)s is not an even number'),
            params={'value': value},
        )

Custom field

--When creating a custom field, you need to implement the ** deconstruct ** method. -** deconstruct ** has a role similar to ** \ _ \ _ eq \ _ \ _ ** above --That is, it returns information to compare whether the Field has changed. --Information is field name (name), field path (path), Field argument to constructor (args), keyword argument (kwargs)

class HandField(models.Field):

    def __init__(self, *args, **kwargs):
        kwargs['max_length'] = 104
        super(HandField, self).__init__(*args, **kwargs)

    def deconstruct(self):  # <- deconstruct
        name, path, args, kwargs = super(HandField, self).deconstruct()
        del kwargs["max_length"]
        return name, path, args, kwargs

# refs https://docs.djangoproject.com/ja/1.9/howto/custom-model-fields/#field-deconstruction

Summary

--Custom validators and special work to create custom fields are required --** deconstructible ** is also required when creating a custom storage class

reference

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