Django - 给定查询集的相关 ManyToManyField 中的实例数

Question

我想知道有多少地图通过 ManyToManyField 与查询集相关，或者换一种说法，有多少地图至少有一个 shapefile 作为图层。

models.py：

class Map(models.Model):
    layers = models.ManyToManyField(Shapefile)

class Shapefile(models.Model):
    filename = models.CharField(max_length=255)

views.py：

shapefiles = Shapefile.objects.filter(created_by=request.user)
map_instances = Map.objects... ???

示例：

shape1 = Shapefile(filename = 'shape1')
shape2 = Shapefile(filename = 'shape2')
shape3 = Shapefile(filename = 'shape3')
map1 = Map(name = 'map1')

map1.layers.add(shape1)
map1.layers.add(shape2)
map1.layers.add(shape3)

shapefiles = Shapefile.objects.all()
map_instance = [shape.map_set.count() for shape in shapefiles]
print map_instance
>>>[1,1,1]
print sum(map_instance)
>>> 3

或者我可以这样做：

map_instance = Map.objects.filter(layers__in=shapefiles)
print map_instance.count()
>>>3

根据我的需要，map_instance 应该返回 1 个地图，因为只有一个地图实例包含 3 个 shapefile。我只是不知道如何使它工作......

Answer 1

也许这就是你要找的东西：

from django.db.models import Count

maps_with_one_or_more_layer = Map.objects.all().annotate(Count('layers')).filter(layers__count__gte=1)

how_many = maps_with_one_or_more_layers.count()

文档：QuerySet API reference - Annotate

Answer 2

map_instances = shapefiles.count()

但是为了更符合 Django 的风格，你可以为你的模型创建一个管理器：

class ShapefileManager(models.Manager):
    def map_instances(self, user):
        return self.filter(created_by=user).count()

class Shapefile(models.Model):
    filename = models.CharField(max_length=255)

    objects = ShapefileManager()

number = Shapefile.objects.map_instances(request.user)

编辑：抱歉，误解了你的问题。但我认为这个链接是你需要的，这是一个类似的问题：

1. count values from manytomanyfield

2. How to count and display objects in relation ManyToMany in Django