how to override save method of models

Question

I am trying to overwrite the save method of models so that when a new book is created it is saved and exported immediately to a file. json, this would be a form of automatic backup, but when registering a new book it exports all the items except the last one registered, but it should export all, someone could give me an example of how to overwrite this method correctly??

def save(self, filename="books", *args, **kwargs):
    get = serializers.serialize("json", Books.objects.all())
    bkp = open("backup/" + filename + ".json", "w")
    bkp.write(get)
    super(Books, self).save(*args, **kwargs)

Answer 1

It is not exporting the last registered because the method save() inside the model works as pre_save. To solve this problem you must use Signal post_save.

from django.dispatch import receiver
from django.db.models.signals import post_save

@receiver(post_save, sender=Book)
def faz_backup_de_livros(sender, instance, created, **kwargs):
    filename = instance.filename  # getattr(instance, 'filename', 'backup')
    get = serializers.serialize("json", Books.objects.all())
    bkp = open("backup/" + filename + ".json", "w")
    bkp.write(get)

After you save a book, you will be called the method faz_backup_de_livros which will save the backup file.

book = Book()
book.titulo = 'Meu livro'
book.filename = 'meu_livro'
book.save()

You can learn more about Signals here: https://docs.djangoproject.com/es/1.9/ref/signals/#post-save

Answer 2

Orion’s response is very beautiful and I fully agree with his solution. However, part of your doubt is how to properly override the save method of a Model subclass.

What you do is right, there’s nothing wrong with your envelope. Only, for you to get the last book saved, you must call the parent class method before backup:

def save(self, filename="books", *args, **kwargs):
    super(Books, self).save(*args, **kwargs)
    get = serializers.serialize("json", Books.objects.all())
    bkp = open("backup/" + filename + ".json", "w")
    bkp.write(get)

The way you wrote, the object is only saved after backup, so it gets incomplete.

Now, man, I need to tell you something, you ever think about what’s gonna happen to your bank if a lot of people save books at the same time? Also, if the number of data in the database is too large, don’t you think the response time for each book creation/update will be too large? What’s more, what happens to the file if two books are saved at the same time?