Django - objects.values（）和prefetch_related（）在同一個查詢中

Question

我有Book，Profile，Book_stat模型如下。我試圖最小化Book模型和prefetch_related方法的字段，以從Book_stat模型中獲取數據。

models.py

class Book(models.Model): 
    title = models.CharField(max_length=200) 
    image = models.FileField(upload_to="mp/", blank=True) 

    def __str__(self): 
     return self.title 

class Profile(models.Model): 
    user = models.OneToOneField(User, on_delete=models.CASCADE) 
    full_name = models.CharField(max_length=150) 

    def __str__(self): 
     return self.full_name 

class Book_stat(models.Model): 
    user = models.ForeignKey(User) 
    book = models.ForeignKey(Book) 
    rating = models.DecimalField(max_digits=5, decimal_places=2, default=Decimal('0.00')) 
    like = models.BooleanField(default=False) 

    def __str__(self): 
     return self.book.title + ' by: ' + self.user.profile.full_name 

views.py

def books(request): 
    books = books = Book.objects.prefetch_related('book_stat_set').values('id','title', 'image') 
    for book in books: 
     for book_stat in book.book_stat_set.all(): # error: 'dict' object has no attribute 'book_stat_set' 
      book.stats = book_stat 
      pprint(vars(book_stat)) 
    return HttpResponse(books, content_type= 'json') 

1）我想的json response發送到前端如下
2）如果可能，我想用book_stat_set.values('like', 'rating')，而不是book_stat_set.all()，使我查詢like，rating只有

books = [ 
    { 
     id: 1, 
     title: 'some title1', 
     image: 'image1.jpg', 
     stats: { 
      like: true, 
      rating: 3.50 
     } 
    }, 
    { 
     id: 2, 
     title: 'some title2', 
     image: 'image1.jpg', 
     stats: { 
      like: true, 
      rating: 3.50 
     } 
    } 
] 

Answer 1

取而代之的是values方法可以使用only和手動序列化：

from django.db.models import Prefetch 
from django.forms import model_to_dict 
from django.http import JsonResponse 

def books(request): 
    prefetch = Prefetch('book_stat_set', queryset=Book_stat.objects.only('like', 'rating')) 
    qs = Book.objects.prefetch_related(prefetch).only('id','title', 'image') 
    result = [] 
    for book in qs: 
     row = model_to_dict(book, fields=['id','title', 'image']) 
     row['image'] = row['image'].url 
     stats = [] 
     for book_stat in book.book_stat_set.all(): 
      stat = model_to_dict(book_stat, fields=['like', 'rating']) 
      stat['rating'] = str(stat['rating']) # or float(stat['rating']) - depends on your purposes 
      stats.append(stat) 
     # you can calculate an average rating here 
     row['stats'] = stats 
     result.append(row) 
    return JsonResponse(result) 

Answer 2

如果你需要它只有一次，你可以用手把它序列化。

books = Book.objects.prefetch_related('book_stat_set') 

books_json = [{ 
    'id': book.id, 
    'title': book.title, 
    'image': book.image.url, 
    'stats': [ 
     { 
      'like': stat.like, 'rating': str(stat.rating) 
     } for stat in book.book_stat_set.all() 
     ] 
} for book in books]