Django count objects field

Question

I need to create a field that count the number of registered books of each own library.

Each book is related to a library by a ForeighKey

If I register or delete a book it should update automatically in a "book_count" field related to your own library.

libraries/models.py

from django.db import models

class Libraries(models.Model):
    name = models.CharField(max_length=50)
    book_count = models.IntegerField(default=0)

    def __str__(self):
        return self.name

books/models.py

from django.db import models

from libraries.models import Libraries

class Books(models.Model):
    library = models.ForeignKey(Libraries, on_delete=models.DO_NOTHING, blank=True, null=True)
    book = models.CharField(max_length=200, null=True, blank=True)

    def __str__(self):
        return self.book

How can I create this simple book counter in the model of each library?

I would use the model name Library for most contexts. Also you have a circular import. — Kent Shikama
– Kent Shikama, Commented Nov 27, 2019 at 21:45
Does this answer your question? Django - Circular model import issue — Kent Shikama
– Kent Shikama, Commented Nov 27, 2019 at 21:45
I resolved the circular model problem, but the counter didn't work. I edited the post to make it easier. — Felipe
– Felipe, Commented Nov 27, 2019 at 22:19
Now have no error message. But I need know How can I create a field to book count, in the model of each library? — Felipe
– Felipe, Commented Nov 27, 2019 at 22:42

Kent Shikama · Accepted Answer · 2019-11-28 03:51:00Z

2

Given the following app

$ tree -L 2
.
├── db.sqlite3
├── libraryapp
│   ├── __init__.py
│   ├── __pycache__
│   ├── admin.py
│   ├── migrations
│   ├── models.py
│   ├── settings.py
│   ├── urls.py
│   └── wsgi.py
└── manage.py

and your models (I've done a small bit of refactoring)

$ cat libraryapp/models.py 
from django.db import models

class Library(models.Model):
    name = models.CharField(max_length=50)
    book_count = models.IntegerField(default=0)

    def __str__(self):
        return self.name

class Book(models.Model):
    name = models.CharField(max_length=200, null=True, blank=True)
    library = models.ForeignKey(Library, on_delete=models.DO_NOTHING, blank=True, null=True)

    def __str__(self):
        return self.name

    def save(self):
        super().save()
        self.update_book_count()

    def delete(self):
        super().delete()
        self.update_book_count()

    def update_book_count(self):
        if self.library:
            self.library.book_count = self.library.book_set.count()
            self.library.save()

you can get the count of books in a certain library as follows:

$ python3 manage.py shell
>>> from libraryapp.models import Book, Library
>>> library = Library.objects.first()
>>> library.book_count
1

Note that this assumes the only way to update the book count of a library is through adding or deleting a book.

edited Nov 28, 2019 at 3:51

answered Nov 27, 2019 at 23:09

Kent Shikama

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6 Comments

Felipe Over a year ago

I can not do this refactoring. I am already finalizing the project and it would be a very difficult change. And "book_count" must be in a field of libreries/models.py. It will be register in DB.

Kent Shikama Over a year ago

You could make book_count a property. See my edit above.

Felipe Over a year ago

But you exclude "book_count" field from Library class. Didn't work. Because I need the IntegerField to be automatically completed/edit by the book count.

Kent Shikama Over a year ago

Fair enough. Please see my edit above. Note that I find this less elegant of a design but given how I'm unaware of the rest of the app maybe it is necessary.

Felipe Over a year ago

I got it using Signals.

|

Felipe · Accepted Answer · 2019-11-28 03:23:27Z

1

I got it using Signals.

@receiver(post_save, sender=Book)
def add_book_count(instance, created, **kwargs):
    if created:
        instance.library.book_count += 1
        instance.library.save()


@receiver(post_delete, sender=Book)
def rem_statement_count(instance, **kwargs):
    instance.library.book_count -= 1
    instance.library.save()

answered Nov 28, 2019 at 3:23

Felipe

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