如何从 Django 中的一组对象中获取平均值？

Question

我有一个简单的房产评级系统。您给它打分，满分 5 分（星）。模型的定义如下：

def Property(models.Model)
    # stuff here

def Rating(models.Model)
    property = models.ForeignKey(Property)
    stars = models.IntegerField()

我想要做的是获取一个属性，找到所有评级对象，收集它们，然后从中获取平均“星星”。

有什么想法如何做到这一点？

Answer 1

您应该使用 Aggregation(doc)：

from django.db.models import Avg

p = Property.objects.get(...)
stars_average = p.rating_set.aggregate(Avg('stars')).values()[0]

有点不确定我的例子虽然。

Answer 2

您可以使用 aggregate() 和 annotate() 与 < a href="https://docs.djangoproject.com/en/4.1/ref/models/querysets/#avg" rel="nofollow noreferrer">Avg() 来平均 Rating 中的星星 模型和属性按属性如下所示。 *我需要将 order_by('pk') 与 annotate() 一起使用，否则值按降序打印：

from django.db.models import Avg

# Average the stars in "Rating" model
print(Rating.objects.aggregate(Avg('stars'))) 

print()

# Average the stars in "Rating" model property by property
for obj in Property.objects.all(): 
    print(
        Rating.objects.filter(property=obj)
                      .aggregate(Avg('stars'))
    )

print()

# Average the stars in "Rating" model property by property
for obj in Property.objects.all(): 
    print(obj.rating_set.aggregate(Avg('stars')))

print()

# Average the stars in "Rating" model property by property
qs = Property.objects.annotate(Avg('rating__stars')).order_by('pk')
for obj in qs:
    print(obj.rating__stars__avg)

然后，以下内容将输出在控制台上：

{'stars__avg': 3.7}

{'stars__avg': 3.2}
{'stars__avg': 3.6}
{'stars__avg': 4.0}

{'stars__avg': 3.2}
{'stars__avg': 3.6}
{'stars__avg': 4.0}

3.2
3.6
4.0

并且，您可以将 stars 列的默认键 stars__avg 更改为 starsAvg ，如下所示：

from django.db.models import Avg

# Average the stars in "Rating" model
print(Rating.objects.aggregate(starsAvg=Avg('stars'))) 
                               # ↑ Here
print()

# Average the stars in "Rating" model property by property
for obj in Property.objects.all(): 
    print(
        Rating.objects.filter(property=obj)
                      .aggregate(starsAvg=Avg('stars'))
    )                            # ↑ Here

print()

# Average the stars in "Rating" model property by property
for obj in Property.objects.all(): 
    print(obj.rating_set.aggregate(starsAvg=Avg('stars')))
                                   # ↑ Here
print()

# Average the stars in "Rating" model property by property
qs = Property.objects.annotate(starsAvg=Avg('rating__stars')).order_by('pk')
for obj in qs:                 # ↑ Here
    print(obj.starsAvg)
              # ↑ Here

然后，默认键更改如下：

{'starsAvg': 3.7}

{'starsAvg': 3.2}
{'starsAvg': 3.6}
{'starsAvg': 4.0}

{'starsAvg': 3.2}
{'starsAvg': 3.6}
{'starsAvg': 4.0}

3.2
3.6
4.0