[英]Django Queryset with annotate
我正在 Django Manager 模型中編寫一種方法。 我想編寫一種方法來找出每位作者的所有已售副本(書籍)的數量。
我有兩個用Manager編寫的模型和方法。 我的問題是該方法也應該可以從任何作者查詢集鏈接,例如
Author.objects.filter(...).exlucde(...).total_copies_sold()
也應該工作。
例子:
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
在我的代碼下面。 它的工作原理類似於上面的示例,但隨后我嘗試了以下操作:
author_books = Author.objects.filter(id=2).total_copies_sold()
我有
'QuerySet' 對象沒有屬性 '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')
[編輯]
謝謝 schillingt 的回復。 我補充說:
class AuthorQueryset(models.QuerySet):
def total_copies_sold(self):
return self.annotate(copies=Sum('books__copies_sold'))
我試過類似的東西:
author_books = Author.objects.filter(id=2).total_copies_sold()
>>> author_books.copies
我有
“AuthorQueryset”對象沒有“副本”屬性
您需要使用Manager.from_queryset
來設置您的經理。 這是文檔。
class AuthorQueryset(models.QuerySet):
def total_copies_sold(self):
...
class Author(models.Model):
objects = models.Manager.from_queryset(AuthorQueryset)()
你要找的是:
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')
現在可以鏈接查詢,例如:
author_total_books = Author.objects.total_copies_sold().first()
但是,您將無法在 QuerySet 對象上使用它,例如:
author_books = Author.objects.filter(id=2).total_copies_sold()
那是因為您正在注釋 Author 對象,而不是 QuerySet。 要獲得該結果,您應該執行:
Author.objects.annotate_with_copies_sold().get(id=2)
author.copies_sold
15
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):
# Write your solution here
return self.annotate(copies_sold=Coalesce(Sum('books__copies_sold'), 0))
class Author(models.Model):
# Make sure this manager is available.
objects = AuthorManager()
# objects = models.Manager.from_queryset(AuthorQuerySet)()
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')
enter code here
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('Book_Author__copies_sold'))
class Author(models.Model):
first_name = models.CharField(max_length=50)
last_name = models.CharField(max_length=50)
objects = AuthorManager()
class Meta:
verbose_name_plural = "Author"
verbose_name = 'Author'
ordering = ('first_name','last_name')
class Book(models.Model):
title = models.CharField(max_length=250, unique=True)
copies_sold = models.PositiveIntegerField(default=0)
author = models.ForeignKey(Author, on_delete=models.CASCADE, related_name='Book_Author')
class Meta:
verbose_name_plural = "Book"
verbose_name = 'Book'
ordering = ('title','copies_sold')
from vehicles.models import Author, Book
try:
author = Author.objects.get(first_name='Mark', last_name='Twain')
except Author.DoesNotExist:
author = Author.objects.create(first_name='Mark', last_name='Twain')
try:
book = Book.objects.get(author=author, title='Adventrure of Huckleberry Finn', copies_sold=7)
except Book.DoesNotExist:
book = Book.objects.create(author=author, title='Adventrure of Huckleberry Finn', copies_sold=7)
pass
try:
book = Book.objects.get(author=author, title='Adventrure of Tomm Saywer', copies_sold=4)
except Book.DoesNotExist:
book = Book.objects.create(author=author, title='Adventrure of Tomm Saywer', copies_sold=4)
pass
author = Author.objects.annotate_with_copies_sold().first()
print(author.copies_sold)
11
1.Create AuthorManager, AuthorQuerySet classes from Author and Books
2.Create Author, Book models
3.Prepare Test Data and use model manager to filter the queryset
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