I am writing one method in Django Manager model. I want to write method that finds out number of all sold copies (books) per author.
I have two models and method written in Manager. My problem is that method should also be chainable from any Author queryset, for example something like
Author.objects.filter(...).exlucde(...).total_copies_sold()
should also work.
Example:
author = Author.objects.create(...)
Book.objects.create(..., author=author, copies_sold=10)
Book.objects.create(..., author=author, copies_sold=20)
author_total_books = Author.objects.total_copies_sold().first()
>>> author_total_books.copies
30
Below my code. It works like in example above, but then I try something like:
author_books = Author.objects.filter(id=2).total_copies_sold()
I got
'QuerySet' object has no attribute 'annotate'
class AuthorManager(models.Manager):
def total_copies_sold(self):
return self.get_queryset().annotate(copies=Sum('book__copies_sold')
class Author(models.Model):
first_name = models.CharField(max_length=120)
last_name = models.CharField(max_length=120)
objects = AuthorManager()
class Book(models.Model):
title = models.CharField(max_length=120)
copies_sold = models.PositiveIntegerField()
author = models.ForeignKey(Author, on_delete=models.CASCADE, related_name='books')
[Edited]
Thank you schillingt for reply. I added:
class AuthorQueryset(models.QuerySet):
def total_copies_sold(self):
return self.annotate(copies=Sum('books__copies_sold'))
I tried something like:
author_books = Author.objects.filter(id=2).total_copies_sold()
>>> author_books.copies
I got
'AuthorQueryset' object has no attribute 'copies'
What you are lookig for is :
from django.db import models
from django.db.models import Sum
from django.db.models.functions import Coalesce
class AuthorManager(models.Manager):
def get_queryset(self):
return AuthorQuerySet(self.model, using=self._db)
def annotate_with_copies_sold(self):
return self.get_queryset().annotate_with_copies_sold()
class AuthorQuerySet(models.QuerySet):
def annotate_with_copies_sold(self):
return self.annotate(copies_sold=Sum('books__copies_sold'))
class Author(models.Model):
objects = AuthorManager()
first_name = models.CharField(max_length=30)
last_name = models.CharField(max_length=30)
class Book(models.Model):
title = models.CharField(max_length=30)
copies_sold = models.PositiveIntegerField()
author = models.ForeignKey(Author, on_delete=models.CASCADE, related_name='books')
Now it is possible to chain queries e.g.:
author_total_books = Author.objects.total_copies_sold().first()
However you will no be able to use it on QuerySet object like:
author_books = Author.objects.filter(id=2).total_copies_sold()
That is because you are annotating Author object, not a QuerySet. To obtain that result you should execute:
Author.objects.annotate_with_copies_sold().get(id=2)
author.copies_sold
15