如何在django (2.1)中添加自定义选择字段？

from django.db import models

class CustomChoiceField(models.Field):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)

    def deconstruct(self):
        name, path, args, kwargs = super().deconstruct()
        return name, path, args, kwargs

    def db_type(self, connection):
        return 'varchar(255)'  # 自定义字段类型在数据库中的存储类型

    def from_db_value(self, value, expression, connection):
        # 从数据库中取出的值转换为Python对象
        return value

    def to_python(self, value):
        # 将值转换为Python对象
        return value

    def get_prep_value(self, value):
        # 准备存储到数据库的值
        return value

    def value_to_string(self, obj):
        # 将字段值转换为字符串
        value = self.value_from_object(obj)
        return self.get_prep_value(value)

from django.db import models

class MyModel(models.Model):
    custom_field = CustomChoiceField(choices=[('option1', 'Option 1'), ('option2', 'Option 2')])

python manage.py makemigrations
python manage.py migrate