我在理解django的模型字段是如何工作方面遇到了问题。我想要实现的是像PriceField（DecimalField），动态创建/注入另一个模型字段，让我们说一个货币（CharField）字段。
我在https://blog.elsdoerfer.name/2008/01/08/fuzzydates-or-one-django-model-field-multiple-database-columns/阅读了有关此主题的有趣博客文章。我认为（并希望）我已经理解了文章的核心信息。但由于其中大部分都有点过时，我不知道它们是否仍然适用于当前的django版本和我的代码。
我使用Django 1.11.4，Python 3.6.2以及使用./manage.py startapp testing创建的干净应用。 models.py中的代码：
from django.db import models
from django.db.models import signals
_currency_field_name = lambda name: '{}_extension'.format(name)
class PriceField(models.DecimalField):
def contribute_to_class(self, cls, name):
# add the extra currency field (CharField) to the class
if not cls._meta.abstract:
currency_field = models.CharField(
max_length=3,
editable=False,
null=True,
blank=True
)
cls.add_to_class(_currency_field_name(name), currency_field)
# add the original price field (DecimalField) to the class
super().contribute_to_class(cls, name)
# TODO: set the descriptor
# setattr(cls, self.name, FooDescriptor(self))
class FooModel(models.Model):
price = PriceField('agrhhhhh', decimal_places=3, max_digits=10, blank=True, null=True)
如果我尝试为该模型创建迁移，则会出现问题。如果显示以下消息执行python manage.py makemigrations：
Migrations for 'testing':
testing/migrations/0001_initial.py
- Create model FooModel
迁移文件0001_initial.py具有以下内容：
# Generated by Django 1.11.4 on 2017-09-11 18:02
from __future__ import unicode_literals
from django.db import migrations, models
import testing.models
class Migration(migrations.Migration):
initial = True
dependencies = [
]
operations = [
migrations.CreateModel(
name='FooModel',
fields=[
('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
('price', testing.models.PriceField(blank=True, decimal_places=3, max_digits=10, null=True, verbose_name='agrhhhhh')),
('price_extension', models.CharField(blank=True, editable=False, max_length=3, null=True)),
],
),
]
对我来说，到目前为止看起来还不错。但是，如果我执行 ./manage.py migrate testing ，django会喊：
Operations to perform:
Apply all migrations: testing
Running migrations:
Applying testing.0001_initial...Traceback (most recent call last):
File "/usr/local/var/pyenv/versions/stockmanagement-3.6.2/lib/python3.6/site-packages/django/db/backends/utils.py", line 63, in execute
return self.cursor.execute(sql)
File "/usr/local/var/pyenv/versions/stockmanagement-3.6.2/lib/python3.6/site-packages/django/db/backends/sqlite3/base.py", line 326, in execute
return Database.Cursor.execute(self, query)
sqlite3.OperationalError: duplicate column name: price_extension
如果迁移文件中只定义了一个此类字段，为什么在重复列名称：price_extension 时出错？这个重复字段来自哪里，是否有针对这种情况的修复？谢谢！
此异常不仅发生在现有数据库中，而且发生在我从头开始使用空数据库时（删除SQLite文件）。迁移命令失败后，这是DB的结构：
./manage.py dbshell
sqlite> .tables
django_migrations
sqlite> .schema
CREATE TABLE "django_migrations" ("id" integer NOT NULL PRIMARY KEY AUTOINCREMENT, "app" varchar(255) NOT NULL, "name" varchar(255) NOT NULL, "applied" datetime NOT NULL);
sqlite> select * from django_migrations;
sqlite>
完整的堆栈跟踪：
./manage.py migrate testing
Operations to perform:
Apply all migrations: testing
Running migrations:
Applying testing.0001_initial...Traceback (most recent call last):
File "/usr/local/var/pyenv/versions/stockmanagement-3.6.2/lib/python3.6/site-packages/django/db/backends/utils.py", line 63, in execute
return self.cursor.execute(sql)
File "/usr/local/var/pyenv/versions/stockmanagement-3.6.2/lib/python3.6/site-packages/django/db/backends/sqlite3/base.py", line 326, in execute
return Database.Cursor.execute(self, query)
sqlite3.OperationalError: duplicate column name: price_extension
The above exception was the direct cause of the following exception:
Traceback (most recent call last):
File "./manage.py", line 22, in <module>
execute_from_command_line(sys.argv)
File "/usr/local/var/pyenv/versions/stockmanagement-3.6.2/lib/python3.6/site-packages/django/core/management/__init__.py", line 363, in execute_from_command_line
utility.execute()
File "/usr/local/var/pyenv/versions/stockmanagement-3.6.2/lib/python3.6/site-packages/django/core/management/__init__.py", line 355, in execute
self.fetch_command(subcommand).run_from_argv(self.argv)
File "/usr/local/var/pyenv/versions/stockmanagement-3.6.2/lib/python3.6/site-packages/django/core/management/base.py", line 283, in run_from_argv
self.execute(*args, **cmd_options)
File "/usr/local/var/pyenv/versions/stockmanagement-3.6.2/lib/python3.6/site-packages/django/core/management/base.py", line 330, in execute
output = self.handle(*args, **options)
File "/usr/local/var/pyenv/versions/stockmanagement-3.6.2/lib/python3.6/site-packages/django/core/management/commands/migrate.py", line 204, in handle
fake_initial=fake_initial,
File "/usr/local/var/pyenv/versions/stockmanagement-3.6.2/lib/python3.6/site-packages/django/db/migrations/executor.py", line 115, in migrate
state = self._migrate_all_forwards(state, plan, full_plan, fake=fake, fake_initial=fake_initial)
File "/usr/local/var/pyenv/versions/stockmanagement-3.6.2/lib/python3.6/site-packages/django/db/migrations/executor.py", line 145, in _migrate_all_forwards
state = self.apply_migration(state, migration, fake=fake, fake_initial=fake_initial)
File "/usr/local/var/pyenv/versions/stockmanagement-3.6.2/lib/python3.6/site-packages/django/db/migrations/executor.py", line 244, in apply_migration
state = migration.apply(state, schema_editor)
File "/usr/local/var/pyenv/versions/stockmanagement-3.6.2/lib/python3.6/site-packages/django/db/migrations/migration.py", line 129, in apply
operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
File "/usr/local/var/pyenv/versions/stockmanagement-3.6.2/lib/python3.6/site-packages/django/db/migrations/operations/models.py", line 97, in database_forwards
schema_editor.create_model(model)
File "/usr/local/var/pyenv/versions/stockmanagement-3.6.2/lib/python3.6/site-packages/django/db/backends/base/schema.py", line 303, in create_model
self.execute(sql, params or None)
File "/usr/local/var/pyenv/versions/stockmanagement-3.6.2/lib/python3.6/site-packages/django/db/backends/base/schema.py", line 120, in execute
cursor.execute(sql, params)
File "/usr/local/var/pyenv/versions/stockmanagement-3.6.2/lib/python3.6/site-packages/django/db/backends/utils.py", line 80, in execute
return super(CursorDebugWrapper, self).execute(sql, params)
File "/usr/local/var/pyenv/versions/stockmanagement-3.6.2/lib/python3.6/site-packages/cachalot/monkey_patch.py", line 113, in inner
out = original(cursor, sql, *args, **kwargs)
File "/usr/local/var/pyenv/versions/stockmanagement-3.6.2/lib/python3.6/site-packages/django/db/backends/utils.py", line 65, in execute
return self.cursor.execute(sql, params)
File "/usr/local/var/pyenv/versions/stockmanagement-3.6.2/lib/python3.6/site-packages/django/db/utils.py", line 94, in __exit__
six.reraise(dj_exc_type, dj_exc_value, traceback)
File "/usr/local/var/pyenv/versions/stockmanagement-3.6.2/lib/python3.6/site-packages/django/utils/six.py", line 685, in reraise
raise value.with_traceback(tb)
File "/usr/local/var/pyenv/versions/stockmanagement-3.6.2/lib/python3.6/site-packages/django/db/backends/utils.py", line 63, in execute
return self.cursor.execute(sql)
File "/usr/local/var/pyenv/versions/stockmanagement-3.6.2/lib/python3.6/site-packages/django/db/backends/sqlite3/base.py", line 326, in execute
return Database.Cursor.execute(self, query)
django.db.utils.OperationalError: duplicate column name: price_extension
可以在https://github.com/hetsch/django_testing下找到包含上述代码的git存储库。如果克隆此存储库（没有任何数据库的干净项目），调用makemigrations然后调用migrate，也会发生此错误。
答案 0 :(得分：2)
根据Django机票＃22555 https://code.djangoproject.com/ticket/22555，这种添加字段的方法不受官方支持。尽管如此，我还是通过以下简单的修复工作：
extension UIButton {
func roundCorners(_ corners:UIRectCorner, radius: CGFloat) {
let path = UIBezierPath(roundedRect: self.bounds, byRoundingCorners: corners, cornerRadii: CGSize(width: radius, height: radius))
let mask = CAShapeLayer()
mask.path = path.cgPath
self.layer.mask = mask
}
}
//Call this Extension like this:
btn.roundCorners([.topLeft, .bottomRight], radius: 20.0)