python - Django:动态模型字段和迁移

Question

我无法理解 Django 模型字段的工作原理。我想要实现的是类似于 PriceField (DecimalField) 的东西，它动态创建/注入(inject)另一个模型字段，比方说货币 (CharField) 字段。

我在 https://blog.elsdoerfer.name/2008/01/08/fuzzydates-or-one-django-model-field-multiple-database-columns/ 上阅读了关于此主题的有趣博客文章.我认为(并希望)我已经理解了这些文章的核心信息。但由于它们中的大多数都有点过时，我不知道它们是否仍然适用于当前的 django 版本和我的以下代码。

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，就会出现问题。如果执行 python manage.py makemigrations，将显示以下消息:

Migrations for 'testing':
  testing/migrations/0001_initial.py
    - Create model FooModel

迁移文件 0001_initial.py 包含以下内容:

# 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)),
            ],
        ),
    ]

到目前为止，对我来说这看起来还不错。但是如果我然后执行 ./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

当迁移文件中只定义了一个这样的字段时，为什么它会在重复的列名:price_extension 上出错？这个重复的字段是从哪里来的，这种情况有没有解决办法？谢谢!

编辑1

此异常不仅发生在已经存在的数据库中，而且当我从头开始使用空数据库(删除 SQLite 文件)时也会发生。迁移命令失败后，这是数据库的结构:

./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>

和完整的堆栈跟踪:

./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

编辑2

可以在以下位置找到包含上述代码的 git 存储库:https://github.com/hetsch/django_testing .如果克隆此存储库(没有任何 DB 的干净项目)、调用 makemigrations 然后调用 migrate，也会发生此错误。

Answer 1

根据 Django 票 #22555 https://code.djangoproject.com/ticket/22555 ，官方不支持这种添加字段的方法。尽管如此，我还是通过以下简单修复使其工作:

def contribute_to_class(self, cls, name):
    # add the extra currency field (CharField) to the class
    # and prevent adding another field instance if the
    # field was allready attached.
    if not cls._meta.abstract and not hasattr(cls, _currency_field_name(name)):
        currency_field = models.CharField(
            max_length=3, 
            editable=False,
            null=True, 
            blank=True
        )
        cls.add_to_class(_currency_field_name(name), currency_field)