[英]How to make an average from values of a foreign key in Django?
I have a book model and a review model like this in Django:我在 Django 中有一个像这样的书籍模型和评论模型:
class Book(models.Model):
ratings = models.FloatField()
class Review(models.Model):
book = models.ForeignKey(
Book,
on_delete=models.CASCADE,
related_name='reviews',
)
review = models.CharField(max_length=5000)
score = models.PositiveSmallIntegerField(null=True, blank=True,
validators=[MinValueValidator(0), MaxValueValidator(10)])
How can I create a method in Book to calculate the ratings (which is the average from all Reviews associated to a specific book)?如何在 Book 中创建一种方法来计算评分(这是与特定书籍相关的所有评论的平均值)?
You can implement a property like:您可以实现如下属性:
from django.db.models import Avg
class Book(models.Model):
@property
def ratings(self):
return self.reviews.aggregate(avg_score=Avg('score'))['avg_score']
Or if you need to do this in bulk , you can annotate your queryset:或者,如果您需要批量执行此操作,您可以注释您的查询集:
from django.db.models import Avg
Book.objects.annotate(
avg_rating=Avg('reviews__score')
)
Book
objects that arise from this queryset, will have an extra attribute avg_rating
, that contains the average of the score
of the related Review
s.从此查询集产生的
Book
对象将具有一个额外的属性avg_rating
,其中包含相关Review
的平均score
。
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