将字段更改为ManyToMany时的Django数据迁移

Question

我有一个Django应用程序,我想在其中将ForeignKey中的字段更改为ManyToManyField.我想保留我的旧数据.最简单/最好的流程是什么？如果重要,我使用sqlite3作为我的数据库后端.

如果我对问题的总结不清楚,这是一个例子.说我有两个型号:

class Author(models.Model):  
    author = models.CharField(max_length=100) 

class Book(models.Model):  
    author = models.ForeignKey(Author)  
    title = models.CharField(max_length=100)

假设我的数据库中有很多数据.现在,我想更改Book模型,如下所示:

class Book(models.Model):  
    author = models.ManyToManyField(Author)  
    title = models.CharField(max_length=100) 

我不想"丢失"我之前的所有数据.

完成此任务的最佳/最简单方法是什么？

肯

Answer 1

我意识到这个问题很老,当时数据迁移的最佳选择是使用South.现在Django有自己的migrate命令,过程略有不同.

我已将这些模型添加到一个名为的应用程序中books- 如果不是您的情况,请相应调整.

首先,将字段添加到Booka related_name和至少一个或两个(或者它们将发生冲突):

class Book(models.Model):  
    author = models.ForeignKey(Author, related_name='book')
    authors = models.ManyToManyField(Author, related_name='books')
    title = models.CharField(max_length=100) 

生成迁移:

$ ./manage.py makemigrations
Migrations for 'books':
  0002_auto_20151222_1457.py:
    - Add field authors to book
    - Alter field author on book

现在,创建一个空迁移来保存数据本身的迁移:

./manage.py makemigrations books --empty
    Migrations for 'books':
0003_auto_20151222_1459.py:

并添加以下内容.要准确了解其工作原理,请查看有关Data Migrations的文档.注意不要覆盖迁移依赖项.

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

from django.db import models, migrations


def make_many_authors(apps, schema_editor):
    """
        Adds the Author object in Book.author to the
        many-to-many relationship in Book.authors
    """
    Book = apps.get_model('books', 'Book')

    for book in Book.objects.all():
        book.authors.add(book.author)


class Migration(migrations.Migration):

    dependencies = [
        ('books', '0002_auto_20151222_1457'),
    ]

    operations = [
        migrations.RunPython(make_many_authors),
    ]

现在author从模型中删除字段 - 它应该如下所示:

class Book(models.Model):
    authors = models.ManyToManyField(Author, related_name='books')
    title = models.CharField(max_length=100)

为此创建一个新的迁移,并运行它们:

$ ./manage.py makemigrations
Migrations for 'books':
  0004_remove_book_author.py:
    - Remove field author from book

$ ./manage.py migrate
Operations to perform:
  Synchronize unmigrated apps: messages, staticfiles
  Apply all migrations: admin, auth, sessions, books, contenttypes
Synchronizing apps without migrations:
  Creating tables...
    Running deferred SQL...
  Installing custom SQL...
Running migrations:
  Rendering model states... DONE
  Applying books.0002_auto_20151222_1457... OK
  Applying books.0003_auto_20151222_1459... OK
  Applying books.0004_remove_book_author... OK

就是这样.以前可用的作者book.author应该在您获得的查询集中book.authors.all().