Django AttributeError: 'CharField' 对象在迁移后没有属性 'model'

Question

注意：我使用的是 Django 1.9 和 Python 3.4.2

当我对我的一个模型进行更改、进行迁移 ( ./manage.py makemigrations) 然后尝试应用迁移 ( ./manage.py migrate)时，我遇到了一个奇怪的错误。

还有一个类似的问题在这里，但没有答案，人们要求更多的信息和意见的唯一可能的解决方案似乎是删除迁移文件，并重新开始-但我真的想找出是什么原因造成这问题以及如何正确修复它。

错误：

Operations to perform:
  Apply all migrations: contenttypes, admin, auth, blog, sessions
Running migrations:
  Rendering model states... DONE
  Applying blog.0004_auto_20160103_1321...Traceback (most recent call last):
  File "./manage.py", line 10, in <module>
    execute_from_command_line(sys.argv)
  File "/Users/owen/src/projects/owen/lib/python3.4/site-packages/django/core/management/__init__.py", line 350, in execute_from_command_line
    utility.execute()
  File "/Users/owen/src/projects/owen/lib/python3.4/site-packages/django/core/management/__init__.py", line 342, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/Users/owen/src/projects/owen/lib/python3.4/site-packages/django/core/management/base.py", line 348, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/Users/owen/src/projects/owen/lib/python3.4/site-packages/django/core/management/base.py", line 399, in execute
    output = self.handle(*args, **options)
  File "/Users/owen/src/projects/owen/lib/python3.4/site-packages/django/core/management/commands/migrate.py", line 200, in handle
    executor.migrate(targets, plan, fake=fake, fake_initial=fake_initial)
  File "/Users/owen/src/projects/owen/lib/python3.4/site-packages/django/db/migrations/executor.py", line 92, in migrate
    self._migrate_all_forwards(plan, full_plan, fake=fake, fake_initial=fake_initial)
  File "/Users/owen/src/projects/owen/lib/python3.4/site-packages/django/db/migrations/executor.py", line 121, in _migrate_all_forwards
    state = self.apply_migration(state, migration, fake=fake, fake_initial=fake_initial)
  File "/Users/owen/src/projects/owen/lib/python3.4/site-packages/django/db/migrations/executor.py", line 198, in apply_migration
    state = migration.apply(state, schema_editor)
  File "/Users/owen/src/projects/owen/lib/python3.4/site-packages/django/db/migrations/migration.py", line 123, in apply
    operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
  File "/Users/owen/src/projects/owen/lib/python3.4/site-packages/django/db/migrations/operations/fields.py", line 201, in database_forwards
    schema_editor.alter_field(from_model, from_field, to_field)
  File "/Users/owen/src/projects/owen/lib/python3.4/site-packages/django/db/backends/base/schema.py", line 482, in alter_field
    old_db_params, new_db_params, strict)
  File "/Users/owen/src/projects/owen/lib/python3.4/site-packages/django/db/backends/base/schema.py", line 564, in _alter_field
    old_default = self.effective_default(old_field)
  File "/Users/owen/src/projects/owen/lib/python3.4/site-packages/django/db/backends/base/schema.py", line 210, in effective_default
    default = field.get_db_prep_save(default, self.connection)
  File "/Users/owen/src/projects/owen/lib/python3.4/site-packages/django/db/models/fields/__init__.py", line 728, in get_db_prep_save
    prepared=False)
  File "/Users/owen/src/projects/owen/lib/python3.4/site-packages/django/db/models/fields/__init__.py", line 720, in get_db_prep_value
    value = self.get_prep_value(value)
  File "/Users/owen/src/projects/owen/lib/python3.4/site-packages/django/db/models/fields/__init__.py", line 1112, in get_prep_value
    return self.to_python(value)
  File "/Users/owen/src/projects/owen/lib/python3.4/site-packages/django/db/models/fields/__init__.py", line 1108, in to_python
    return smart_text(value)
  File "/Users/owen/src/projects/owen/lib/python3.4/site-packages/django/utils/encoding.py", line 42, in smart_text
    return force_text(s, encoding, strings_only, errors)
  File "/Users/owen/src/projects/owen/lib/python3.4/site-packages/django/utils/encoding.py", line 76, in force_text
    s = six.text_type(s)
  File "/Users/owen/src/projects/owen/lib/python3.4/site-packages/django/db/models/fields/__init__.py", line 188, in __str__
    model = self.model
AttributeError: 'CharField' object has no attribute 'model'

更改前我的模型：

from django.db import models
from django.utils import timezone


class Category(models.Model):
    title = models.CharField(max_length=100, unique=True)

    def __str__(self):
        return self.title

    def number_of_articles(self):
        articles = Article.objects.filter(categories__pk=self.id)
        return articles.count()


class Article(models.Model):
    title = models.CharField(max_length=255, unique=True)
    slug = models.SlugField(max_length=255, unique=True, default=title)
    pub_date = models.DateTimeField('date published', default=timezone.now)
    intro_text = models.TextField(blank=True)
    body_copy = models.TextField(blank=False)
    votes_helpful = models.IntegerField(default=0, blank=True)
    votes_unhelpful = models.IntegerField(default=0, blank=True)

    categories = models.ManyToManyField(Category, blank=True)

    class Meta:
        ordering = ('-pub_date',)

    def __str__(self):
        return self.title

    def has_votes(self):
        total_num_votes = self.votes_helpful + self.votes_unhelpful
        return total_num_votes > 0

我的模型的变化：

slug = models.SlugField(max_length=255, blank=True)

变化是我已经unique=True, default=title从文章中删除并添加了blank=False.

我的迁移文件：

# -*- coding: utf-8 -*-
# Generated by Django 1.9 on 2016-01-03 13:21
from __future__ import unicode_literals

from django.db import migrations, models


class Migration(migrations.Migration):

    dependencies = [
        ('blog', '0003_auto_20151230_1643'),
    ]

    operations = [
        migrations.AlterField(
            model_name='article',
            name='slug',
            field=models.SlugField(blank=True, max_length=255),
        ),
    ]

Answer 1

slug = models.SlugField(max_length=255, unique=True, default=title)

这为您的默认slug领域的一个CharField实例。那是不正确的，我很惊讶这并没有在早些时候给你一个错误。

我认为唯一的解决方案是更改引入旧默认值的迁移文件，并将默认值更改为有效值。