另一个表的字段返回计数

Question

我需要 read_booksfield（已购买），返回 Rating 对象计数。

应该根据Rating中每本书的同一个集合和同一个用户的过滤器返回对象个数。

我不知道该怎么做，因为Rating 和Purchased 没有直接依赖关系。

评级/models.py

class Rating(models.Model):
    user = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.DO_NOTHING)
    book = models.ForeignKey(Book, on_delete=models.CASCADE, null=False)
    rating = models.IntergerField(default=0)

book/models.py

class Books(models.Model):      
    collection = models.ForeignKey(Collection, on_delete=models.DO_NOTHING, blank=True, null=True)
    book = models.CharField(max_length=250, blank=True, null=True)

collection/models.py

class Collection(models.Model):
    title = models.CharField(max_length=50)
    creator = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.DO_NOTHING)

purchased_collections/models.py

class Purchased(models.Model):
    user = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.DO_NOTHING)
    collection = models.ForeignKey(Collection, on_delete=models.DO_NOTHING)
    date = models.DateTimeField(default=datetime.now) 
    read_books = models.IntegerField(default=0)

应用@ruddra 的回复后

第一个选项：

    def read_books(self):
        return Rating.objects.filter(user=self.user, book__collection=self.collection).count()

曾在models.py 工作。但我只想在用户登录时运行此功能。 所以我试着把它放在views.py中的def login (request):， 但是，它不是基于class，我不能使用（self）。总是返回错误name 'self' is not defined.

所以，我尝试了第二个@ruddra 的建议：

purchases = Purchased.objects.annotate(read_books=Count('collection__books__rating', filter=Q(collection__books__rating__user=F('user'))))
for item in purchases:
    print(item.read_books)

views.py

from django.shortcuts import render, redirect, get_object_or_404
from django.contrib import messages
from .login import login_user

from purchased.models import Purchased
from rating.models import Rating
from collection.models import Collection
from book.models import Books

from django.db.models import F, Count, Q

def login(request):
    if not request.user.is_authenticated:
        if request.method == 'POST':
            login_status = login_user(request)
            if login_status != 1:
                messages.error(request, 'Invalid')
                return redirect('login')
            else:
                messages.success(request, 'You are now logged in')

                purchases = Purchased.objects.annotate(read_books=Count('collection__books__rating', filter=Q(collection__books__rating__user=F('user'))))
                for item in purchases:
                   print(item.read_books)


                return redirect('home')

        else:
            return render(request, 'users/login.html')
    else:
        messages.error(request, 'You are already logged in')
        return redirect('index')

但曾经返回错误：NameError: name 'collection__books__rating__user' is not defined.

问题是能够通过自己的collection 过滤每个ratings 对象。在models.py 中使用 (self) 可以完美运行，但如果没有 (self)，我将无法使其正常运行。

还有什么其他建议可以让这个计数只在用户登录时运行？

Answer 1

假设您的意思是 book = models.ForeignKey(Book, on_delete=models.CASCADE, null=False) in Rating 模型：

可以使用属性方法返回计数：

class Purchased(models.Model):
    user = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.DO_NOTHING)
    collection = models.ForeignKey(Collection, on_delete=models.DO_NOTHING)
    date = models.DateTimeField(default=datetime.now) 

    @property
    def read_books(self):
        return Rating.objects.filter(user=self.user, book__collection=self.collection).count()

或者更好，如果您使用annotation（这将减少数据库命中，您可以使用它进行查询）：

from django.db.models import Count, F, Q

purchases = Purchased.objects.annotate(read_books=Count('collection__books__rating', filter=Q(collection__books__rating__user=F('user'))))

for item in purchases:
    print(item.read_books)

Answer 2

如cmets中所述，如果您使用Book作为外键相关模型，那么您可以通过以下方式找出计数：

class Purchased(models.Model):
    user = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.DO_NOTHING)
    collection = models.ForeignKey(Collection, on_delete=models.DO_NOTHING)
    date = models.DateTimeField(default=datetime.now) 
    read_books = models.IntegerField(default=0)

    def save(self):
        self.read_books = Rating.objects.filter(user=self.user, book__in=self.collection.books_set).count()
        return super().save()