Django ORM 一对多 和 多对多

class Province(models.Model):
    name = models.CharField(max_length=32)

    def __str__(self):
        return self.name


class City(models.Model):
    name = models.CharField(max_length=32)
    pro = models.ForeignKey("Province", to_field='id', on_delete=models.CASCADE)

    def __str__(self):
        return self.name

python manage.py makemigrations

python manage.py migrate

    path('test.html', views.test),

from app01 import models

def test(req):
    # 添加数据
    # models.Province.objects.create(name='河北')
    # models.Province.objects.create(name='广东')
    # models.Province.objects.create(name='山东')
    # models.City.objects.create(name='东莞', pro_id=2)
    # models.City.objects.create(name='深圳', pro_id=2)
    # models.City.objects.create(name='惠州', pro_id=2)
    # models.City.objects.create(name='河源', pro_id=2)
    # models.City.objects.create(name='泰安', pro_id=3)
    # models.City.objects.create(name='青岛', pro_id=3)
    # models.City.objects.create(name='济南', pro_id=3)
    # models.City.objects.create(name='张家口', pro_id=1)
    # models.City.objects.create(name='邢台', pro_id=1)

    # 正向查找（具有外键字段的）
    v = models.City.objects.values('id', 'pro__city', 'pro__city__name', 'pro__name')
    print(v)
    # 截取一部分结果： {'pro__name': '河北', 'pro__city__name': '张家口', 'id': 8, 'pro__city': 8}......

    # 反向查找
    l = models.Province.objects.values('id', 'name', 'city__name')
    print(l)
    # 截取一部分结果： {'id': 1, 'name': '河北', 'city__name': '张家口'} , {'id': 1, 'name': '河北', 'city__name': '邢台'}....

    # 获取 province 所有的值
    pro_list = models.Province.objects.all()
    print(pro_list)
    # 结果：<QuerySet [<Province: 河北>, <Province: 广东>, <Province: 山东>]>

    # 获取所有 省份 并且 城市id 都小于5的值
    for item in pro_list:
         print(item.id, item.name, item.city_set.filter(id__lt=5))
    
    # 获取第一个省份和其所有的城市
    print(pro_list[0])
    print(pro_list[0].city_set.all())
    # 结果：
    # 河北
    # <QuerySet [<City: 张家口>, <City: 邢台>]>

    return HttpResponse('ok')

class Book(models.Model):
    name = models.CharField(max_length=32)


class Author(models.Model):
    name = models.CharField(max_length=32)
    m = models.ManyToManyField('Book')

python manage.py makemigrations

python manage.py migrate

def test(req):
    # 正向查找，获取 Author id 为1 的所有著作
    obj = models.Author.objects.get(id=1)
    l = obj.m.all()
    for item in l:
        print(item.id, item.name)

    # 反向查找，获取 著作为 1 的作者
    obj = models.Book.objects.get(id=1)
    m = obj.author_set.all()
    for item in m:
        print(item.id, item.name)

    return HttpResponse('ok')

    # 或者作者和其对应的所有著作
    author_list = models.Author.objects.all()
    for author in author_list:
        print(author.name, author.m.all())

    # 通过第三张表 m 来查询数据
    author_list = models.Author.objects.values('id', 'name', 'm', 'm__name')
    for item in author_list:
        print(item['id'], item['name'], '书籍ID:', item['m'], item['m__name'])

    # 增加
    obj = models.Author.objects.get(id=1)
    obj.m.add(5)
    obj.m.add(4, 5)
    obj.m.add(*[4, 5])
    
    # 删除
    obj.m.remove(5)
    obj.m.remove(4, 5)
    obj.m.remove(*[4, 5])

    # 清空
    obj.m.clear()

    # 更新
    obj.m.set([1, 2, 3])

    # 添加
    obj = models.Book.objects.get(id=1)
    obj.author_set.add(1)
    obj.author_set.add(1, 2, 3, 4)