Django Prefetch上的多重过滤

根据我的代码库:

class City(models.Model):
    name = models.CharField(max_length=100)
    ...

class District(models.Model):
    city = models.ForeignKey(City, on_delete=models.CASCADE)

class Park(models.Model):
    district = models.ForeignKey(District, on_delete=models.CASCADE, related_name="parks")

class Plant(models.Model):
    name = models.CharField(max_length=200)
    plant_type = models.CharField(max_length=200)
    park = models.ForeignKey(Park, on_delete=models.CASCADE, related_name="plants")

def plants_of_type_in_park(plant_type, park):
    return park.plants.filter(plant_type=plant_type)

def generate_garden_report():
    report_data = {}
    plant_types = ["rose", "tulip", "daisy", ...etc]
    districts = District.objects.all()
    districts = districts.prefetch_related("parks", "parks__plants")
    for dist in districts.all():
        for park in districts.parks.all():
            for plant in plant_types:
                report_data[dist.city.name][plant] += plants_of_type_in_park(plant, park).count()
    return report_data

我的目标是使用预取来优化generate_garden_report函数.我加了districts.prefetch_related("parks", "parks__plants").但是，由于plants_of_type_in_park使用筛选器，因此它会触发另一个查询.我的方法是预取每个过滤器:

districts = districts.prefetch_related("parks", "parks__plants")
for plant in plant_types:
    districts = districts.prefetch_related(Prefetch("parks_plants", queryset=Plant.objects.filter(plant_type=plant)))

但是，由于parks_plants只能分配一次，所以我不能为此创建多个筛选器.我的目标是在对plants_of_type_in_park函数影响最小的情况下预取所需的一切，因为它在代码库的其他地方使用.