Django实战-多对多查询

from django.db import models

class Goods(models.Model):#商品
    name = models.CharField(max_length=20)
    price = models.DecimalField(max_digits=5, decimal_places=2)
    gc = models.ForeignKey("Category", null=True, on_delete=models.SET_NULL)#gc为外键，类别表为母表

class Category(models.Model):#类别
    name = models.CharField(max_length=20)

class Store(models.Model):#商家
    name = models.CharField(max_length=30)
    detail = models.TextField(blank=True, null=True)
    sc = models.ManyToManyField("Category")#与类别表进行多对多关联

# 添加商家
Store.objects.create(name="商家A", detail="物美价廉，限时抄底折扣")
Out[2]: <Store: Store object>

Store(name="商家B", detail="大促销").save()

# 添加类别
Category.objects.create(name="电脑整机")
<Category: Category object>

Category(name="文具").save()

# 创建商家C添加全部分类
Store.objects.create(name="商家C").sc.add(*(Category.objects.all()))#如果商户已存在则把create改成get

store = Store.objects.get(name="商家C")
store.sc=(Category.objects.all())
store.save()

# 创建商家D添加指定分类
store = Store.objects.create(name="商家D")
category = Category.objects.filter(name__in=["电脑整机","文具"])#单个改成get，全部改成all
store.sc.add(*category)#add是追加模式
store.sc.clear()#清空此商家的商品

#让指定商品分类添加指定的商家，反向查询
store = Store.objects.create(name="商家E")
category = Category.objects.get(name="电脑整机")
category.store_set.add(store)

# 让所有商家都添加这个分类
stores = Store.objects.all()
category = Category.objects.get(name="电脑整机")
category.store_set.add(*stores)

#让所有商家去除这个分类
category.store_set.clear()
# 删除store_set的所有商家
category.store_set.all().delete()
#只有子表才有"子表名小写_set"的写法，得到的是一个QuerySet集合，后边可以接.add(),.remove(),.update(),.delete(),.clear()