Django:动态模型字段和迁移

时间：2021-05-20 11:47:52

I have problems understanding how django's model fields work. What I want to achieve is something like a PriceField (DecimalField), that dynamically creates/injects another model field, let's say a currency (CharField) field.

我不理解django的模型字段是如何工作的。我想要实现的是一个PriceField (DecimalField)，它动态地创建/注入另一个模型字段，假设是一个currency (CharField)字段。

I have read an interesing blog posts about this topic at https://blog.elsdoerfer.name/2008/01/08/fuzzydates-or-one-django-model-field-multiple-database-columns/. I think (and hope) that I've understood the core messages of the articles. But as most of them are a little bit outdated, I don't know if they are still valid for current django versions and my below code.

我在https://blog.elsdoerfer.name/2008/01/08/fuzzydate -or-one-django-model-field-multi -database-columns/上读过一篇关于这个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版本和我的下面代码。

I use Django 1.11.4, Python 3.6.2, and a clean app created with ./manage.py startapp testing. The code in models.py:

我使用Django 1.11.4、Python 3.6.2和一个用./manage创建的干净的应用程序。py startapp测试。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)

The problems come if I try to create migrations for that models. If executing python manage.py makemigrations following message is shown:

如果我试图为该模型创建迁移，就会出现问题。如果执行python管理。以下消息显示py移民计划:

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

Migration file 0001_initial.py has the following content:

0001年_initial移民文件。py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)),
            ],
        ),
    ]

For me this looks OK so far. But if I then execute ./manage.py migrate testing, django shouts:

对我来说，到目前为止还好。但如果我执行。/管理。py迁移测试，django喊道:

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

Why does it error out on a duplicate column name: price_extension, when there is only one such field defined in the migrations file? Where does this duplicate field come from and is there a fix for this situation? Thanks!

如果在迁移文件中只定义了一个这样的字段，为什么在重复的列名price_extension上出错呢?这个重复字段从何而来，这种情况有解决办法吗?谢谢!

Edit 1

This exception not only happens with an already existing database but also when I start with an empty database from scratch (deleting SQLite file). After the migrate command failed this is the structure of the DB:

这个异常不仅发生在已经存在的数据库上，而且当我从头开始使用一个空数据库(删除SQLite文件)时也会发生。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>

And full stacktrace:

和加亮:

./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

Edit 2

A git repository with the above code can be found under: https://github.com/hetsch/django_testing. This error happens also if one clones this repository (clean project without any DB), calls makemigrations and then migrate.

可以在以下找到具有上述代码的git存储库:https://github.com/hetsch/django_testing。如果一个人克隆了这个存储库(没有任何DB的清洁项目)，调用了makmigration然后迁移，那么这个错误也会发生。

1 个解决方案

#1

1  

According to Django ticket #22555 https://code.djangoproject.com/ticket/22555, this method of adding fields is not officially supported. Nonetheless, I made it work with the following simple fix:

根据Django ticket #22555 https://code.djangoproject.com/ticket/22555，不支持这种添加字段的方法。尽管如此，我还是做了如下简单的修正:

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)

#1

1  

According to Django ticket #22555 https://code.djangoproject.com/ticket/22555, this method of adding fields is not officially supported. Nonetheless, I made it work with the following simple fix:

根据Django ticket #22555 https://code.djangoproject.com/ticket/22555，不支持这种添加字段的方法。尽管如此，我还是做了如下简单的修正:

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)
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