Django ReverseSingleRelatedObjectDescriptor.__set__ ValueError

Question

我正在创建一个自定义数据迁移，以根据跨两个不同模型的现有条目在数据库中自动创建 GenericRelation 条目。

示例模型.py：

...
class Place
   content_type = models.ForeignKey(ContentType)
   object_id = models.PositiveIntegerField()
   content_object = generic.GenericForeignKey('content_type', 'object_id')


class Restaurant
   name = models.CharField(max_length=60)
   location = models.CharField(max_length=60)


class House
   location = models.CharField(max_length=60)

示例 0011_place_data.py：

# -*- coding: utf-8 -*-
from django.contrib.contenttypes.models import ContentType
from django.db import models, migrations

def forwards_func(apps, schema_editor):
    Restaurant = apps.get_model("simpleapp", "Restaurant")
    House = apps.get_model("simpleapp", "House")
    Place = apps.get_model("simpleapp", "Place")

    db_alias = schema_editor.connection.alias

    content_type = ContentType.objects.using(db_alias).get(
        app_label="simpleapp",
        model="restaurant"
    )

    for restaurant in Restaurant.objects.using(db_alias).all():
        Place.objects.using(db_alias).create(
            content_type=content_type,
            object_id=restaurant.id)

    content_type = ContentType.objects.using(db_alias).get(
        app_label="simpleapp",
        model="house"
    )
    for house in House.objects.using(db_alias).all():
        Place.objects.using(db_alias).create(
            content_type=content_type,
            object_id=house.id)


class Migration(migrations.Migration):

    dependencies = [
        ('simpleapp', '0010_place')
    ]

    operations = [
        migrations.RunPython(
            forwards_func,
        ),
    ]

当我运行这个（Django 1.7.4）时，我得到了

Operations to perform:
  Apply all migrations: simpleapp, admin, sessions, auth, contenttypes
Synchronizing apps without migrations:
  Creating tables...
  Installing custom SQL...
  Installing indexes...
Running migrations:
  Applying projects.0011_place_data...passing
Traceback (most recent call last):
  File "manage.py", line 10, in <module>
    execute_from_command_line(sys.argv)
  File ".../lib/python3.4/site-packages/django/core/management/__init__.py", line 385, in execute_from_command_line
utility.execute()
  File ".../lib/python3.4/site-packages/django/core/management/__init__.py", line 377, in execute
self.fetch_command(subcommand).run_from_argv(self.argv)
  File ".../lib/python3.4/site-packages/django/core/management/base.py", line 288, in run_from_argv
self.execute(*args, **options.__dict__)
  File ".../lib/python3.4/site-packages/django/core/management/base.py", line 338, in execute
output = self.handle(*args, **options)
  File ".../lib/python3.4/site-packages/django/core/management/commands/migrate.py", line 161, in handle
executor.migrate(targets, plan, fake=options.get("fake", False))
  File ".../lib/python3.4/site-packages/django/db/migrations/executor.py", line 68, in migrate
self.apply_migration(migration, fake=fake)
  File ".../lib/python3.4/site-packages/django/db/migrations/executor.py", line 102, in apply_migration
migration.apply(project_state, schema_editor)
  File ".../lib/python3.4/site-packages/django/db/migrations/migration.py", line 108, in apply
operation.database_forwards(self.app_label, schema_editor, project_state, new_state)
  File ".../lib/python3.4/site-packages/django/db/migrations/operations/special.py", line 117, in database_forwards
self.code(from_state.render(), schema_editor)
  File ".../simpleapp/migrations/0011_place_data.py", line 19, in forwards_func
object_id=restaurant.id)
  File ".../lib/python3.4/site-packages/django/db/models/query.py", line 370, in create
obj = self.model(**kwargs)
  File ".../lib/python3.4/site-packages/django/db/models/base.py", line 440, in __init__
setattr(self, field.name, rel_obj)
  File ".../lib/python3.4/site-packages/django/db/models/fields/related.py", line 598, in __set__
self.field.rel.to._meta.object_name,
ValueError: Cannot assign "<ContentType: restaurant>": "Place.content_type" must be a "ContentType" instance.

如果我在 Django 模块 (django.db.models.fields.related.ReverseSingleRelatedObjectDescriptor.set) 中注释掉引发值错误的节，它会按预期工作：

...

    elif value is not None and not isinstance(value, self.field.rel.to):
        print('skipping')
        #raise ValueError(
        #    'Cannot assign "%r": "%s.%s" must be a "%s" instance.' % (
        #        value,
        #        instance._meta.object_name,
        #        self.field.name,
        #        self.field.rel.to._meta.object_name,
        #    )
        #)

...

应该首先提出这个异常吗？这是 Django 中的错误还是我做错了？

Answer 1

编辑：

下面的解决方案实际上不起作用；它只是最终没有运行 forwards_func 所以没有错误。欢迎新的解决方案：

---

我可以使用Django's post_migrate signal 解决这个问题。

这还解决了这些类型的迁移（引用 ContentType 表的数据迁移）带来的一些其他问题。

我的理解是，本质上，问题是出于性能原因，直到迁移结束时才会创建 ContentType 表。这意味着我实际上并没有检索关系模块正在检查的相同类型的 ContentType 对象。

解决方案是将此类数据迁移作为回调运行：

# -*- coding: utf-8 -*-
from django.db.models.signals import post_migrate
from django.contrib.contenttypes.models import ContentType
from django.db import models, migrations

def forwards_func(apps, schema_editor):
    Restaurant = apps.get_model("simpleapp", "Restaurant")
    House = apps.get_model("simpleapp", "House")
    Place = apps.get_model("simpleapp", "Place")

    db_alias = schema_editor.connection.alias

    def add_stuffs(*args, **kwargs)
        content_type = ContentType.objects.using(db_alias).get(
            app_label="simpleapp",
            model="restaurant"
        )

        for restaurant in Restaurant.objects.using(db_alias).all():
            Place.objects.using(db_alias).create(
                content_type=content_type,
                object_id=restaurant.id)

        content_type = ContentType.objects.using(db_alias).get(
            app_label="simpleapp",
            model="house"
        )
        for house in House.objects.using(db_alias).all():
            Place.objects.using(db_alias).create(
               content_type=content_type,
               object_id=house.id)

    post_migrate.connect(add_stuffs)


class Migration(migrations.Migration):

    dependencies = [
        ('simpleapp', '0010_place')
    ]

    operations = [
        migrations.RunPython(
            forwards_func,
        ),
    ]

Answer 2

貌似和这个 Django bug/wontfix有点关系

在我的迁移中尝试为用户组重新分配权限时，我遇到了类似的错误。要修复它，我必须按照here 的描述手动发出 post_migrate 信号

这是我的迁移代码示例：

# -*- coding: utf-8 -*-
from __future__ import unicode_literals

from django.db import migrations
from django.core.management.sql import emit_post_migrate_signal


def trigger_post_migrate(db_alias):
    try:
        # Django 1.9
        emit_post_migrate_signal(2, False, db_alias)
    except TypeError:
        # Django < 1.9
        try:
            # Django 1.8
            emit_post_migrate_signal(2, False, 'default', db_alias)
        except TypeError:
            # Django < 1.8
            emit_post_migrate_signal([], 2, False, 'default', db_alias)


def up(apps, schema_editor):
    # trigger post migrate to make sure all permissions and contenttypes are in place
    trigger_post_migrate(schema_editor.connection.alias)

    # migration code here...


class Migration(migrations.Migration):
    dependencies = [
        ('accounts', '0008_auto_20160712_0608'),
    ]

    operations = [
        migrations.RunPython(up),
    ]