from Django data migration when changing a field to ManyToMany
django rename model (2)
Probably the best and easiest thing you should do would be:
Create the Many to many field with a different name say
authors = models.ManyToManyField(Author)
write a small function to convert foreignkey values to M2M values:
def convert(): books = Book.objects.all() for book in books: if book.author: li = [book.author.id] book.authors.append(li) book.save()
Once it is run, you can delete the author field from the table and run migration again.
I have a Django application in which I want to change a field from a ForeignKey to a ManyToManyField. I want to preserve my old data. What is the simplest/best process to follow for this? If it matters, I use sqlite3 as my database back-end.
If my summary of the problem isn't clear, here is an example. Say I have two models:
class Author(models.Model): author = models.CharField(max_length=100) class Book(models.Model): author = models.ForeignKey(Author) title = models.CharField(max_length=100)
Say I have a lot of data in my database. Now, I want to change the Book model as follows:
class Book(models.Model): author = models.ManyToManyField(Author) title = models.CharField(max_length=100)
I don't want to "lose" all my prior data.
What is the best/simplest way to accomplish this?
I realize this question is old and at the time the best option for Data Migrations was using South. Now Django has its own
migrate command, and the process is slightly different.
I've added these models to an app called
books -- adjust accordingly if that's not your case.
First, add the field to
Book and a
related_name to at least one, or both of them (or they'll clash):
class Book(models.Model): author = models.ForeignKey(Author, related_name='book') authors = models.ManyToManyField(Author, related_name='books') title = models.CharField(max_length=100)
Generate the migration:
$ ./manage.py makemigrations Migrations for 'books': 0002_auto_20151222_1457.py: - Add field authors to book - Alter field author on book
Now, create an empty migration to hold the migration of the data itself:
./manage.py makemigrations books --empty Migrations for 'books': 0003_auto_20151222_1459.py:
And add the following content to it. To understand exactly how this works, check the documentation on Data Migrations. Be careful not to overwrite the migration dependency.
# -*- 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), ]
Now remove the
author field from the Model -- it should look like this:
class Book(models.Model): authors = models.ManyToManyField(Author, related_name='books') title = models.CharField(max_length=100)
Create a new migration for that, and run them all:
$ ./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
And that's it. The authors previously available at
book.author now should be in the queryset you get from