自定义JsonField，用json.dumps转成字串存到db里，从db取出来用json.loads转会真实类型
可以存dict或者list，默认不传的时候存空字串，注意这个字段只能用来存取，不要用来做filter

import json
from django.db import models

class CustomJsonField(models.TextField):
	def get_db_prep_value(self, value, connection, prepared=False):
		"""存到db"""
		if not value:
			return super().get_db_prep_value('', connection, prepared)
		return json.dumps(value, ensure_ascii=False)
	
	def from_db_value(self, value, expression, connection):
		"""从db取出"""
		if not value:
			return ''
		return json.loads(value)

class Person(models.Model):
	name = models.CharField('名字', max_length=255)
	desc = CustomJsonField('说明', default='')

# 创建和调用举例
Person.objects.create(name='zhangsan', desc=[1,2,3])
>>> <Person: Person object (1)>
Person.objects.create(name='lisi', desc={'x':1, 'y':2})
>>> <Person: Person object (2)>
Person.objects.get(id=1).desc
>>> [1, 2, 3]
Person.objects.get(id=2).desc
>>> {'x': 1, 'y': 2}