django-signals.py

#!/usr/bin/env python
# https://docs.djangoproject.com/en/3.1/topics/settings/#calling-django-setup-is-required-for-standalone-django-usage
import django
from django.conf import settings

# https://blog.amedama.jp/entry/sqlite3-in-memory-issue
import tempfile
tfile = tempfile.NamedTemporaryFile()

import sqlite3
conn = sqlite3.connect(tfile.name)
c = conn.cursor()
c.execute('CREATE TABLE book (id integer primary key, title text not null)')
c.execute('CREATE TABLE book_log (id integer primary key, book_id int not null, title text not null)')

settings.configure(DEBUG=True, DATABASES={
    'default': {
        'ENGINE': 'django.db.backends.sqlite3',
        'NAME': tfile.name,
    }
})
django.setup()

# Now this script or any imported module can use any part of Django it needs.
from django.db import models

class Book(models.Model):
    id = models.BigAutoField(primary_key=True)
    title = models.TextField(null=False)

    def __str__(self):
        return self.title

    class Meta:
        app_label = 'standalone'
        db_table = 'book'

class BookLog(models.Model):
    id = models.BigAutoField(primary_key=True)
    book_id = models.BigIntegerField(null=False)
    title = models.TextField(null=False)

    def __str__(self):
        return self.title

    class Meta:
        app_label = 'standalone'
        db_table = 'book_log'

# https://docs.djangoproject.com/ja/3.1/topics/signals/
from django.db.models.signals import pre_save
from django.dispatch import receiver

@receiver(pre_save, sender=Book)
def my_handler(sender, instance, **kwargs):
    # `instance` is a model object to be updated
    old_book = Book.objects.filter(id=instance.id).first()
    if not old_book:
        return

    if old_book.title == instance.title:
        return
    BookLog(book_id=old_book.id, title=old_book.title).save()


book = Book(title="My First Book")
book.save()
print(book)
print(BookLog.objects.count())

book.save()
print(book)
print(BookLog.objects.count())

book.title = "My 1st Book"
book.save()
print(book)
print(BookLog.objects.count())
print(BookLog.objects.all())