django组件：contenttype
组件的作用：可以通过两个字段让表和N张表创建FK关系


表结构：
from django.db import models
from django.contrib.contenttypes.models import ContentType

from django.contrib.contenttypes.fields import GenericForeignKey, GenericRelation

class DegreeCourse(models.Model):
"""学位课程"""
name = models.CharField(max_length=128, unique=True)
course_img = models.CharField(max_length=255, verbose_name="缩略图")
brief = models.TextField(verbose_name="学位课程简介", )

class Course(models.Model):
"""专题课程"""
name = models.CharField(max_length=128, unique=True)
course_img = models.CharField(max_length=255)

# 不会在数据库生成列，只用于帮助你进行查询
policy_list = GenericRelation("PricePolicy")

class PricePolicy(models.Model):
"""价格与有课程效期表"""
content_type = models.ForeignKey(ContentType) # 关联course or degree_course
object_id = models.PositiveIntegerField()

#不会在数据库生成列，只用于帮助你进行添加和查询
content_object = GenericForeignKey('content_type', 'object_id')

valid_period_choices = (
(1, '1天'),
(3, '3天'),
(7, '1周'), (14, '2周'),
(30, '1个月'),
(60, '2个月'),
(90, '3个月'),
(180, '6个月'), (210, '12个月'),
(540, '18个月'), (720, '24个月'),
)
valid_period = models.SmallIntegerField(choices=valid_period_choices)
price = models.FloatField()

使用：
# 1.在价格策略表中添加一条数据
# models.PricePolicy.objects.create(
# valid_period=7,
# price=6.6,
# content_type=ContentType.objects.get(model='course'),
# object_id=1
# )

# models.PricePolicy.objects.create(
# valid_period=14,
# price=9.9,
# content_object=models.Course.objects.get(id=1)
# )

# 2. 根据某个价格策略对象，找到他对应的表和数据，如：管理课程名称
# price = models.PricePolicy.objects.get(id=2)
# print(price.content_object.name) # 自动帮你找到

# 3.找到某个课程关联的所有价格策略
# obj = models.Course.objects.get(id=1)
# for item in obj.policy_list.all():
# print(item.id,item.valid_period,item.price)