Django Change a CharField to a ForeignKey


Example

First off, let's assume this is your initial model, inside an application called discography:

from django.db import models

class Album(models.Model):
    name = models.CharField(max_length=255)
    artist = models.CharField(max_length=255)

Now, you realize that you want to use a ForeignKey for the artist instead. This is a somewhat complex process, which has to be done in several steps.

Step 1, add a new field for the ForeignKey, making sure to mark it as null (note that the model we are linking to is also now included):

from django.db import models

class Album(models.Model):
    name = models.CharField(max_length=255)
    artist = models.CharField(max_length=255)
    artist_link = models.ForeignKey('Artist', null=True)

class Artist(models.Model):
    name = models.CharField(max_length=255)

...and create a migration for this change.

./manage.py makemigrations discography

Step 2, populate your new field. In order to do this, you have to create an empty migration.

./manage.py makemigrations --empty --name transfer_artists discography

Once you have this empty migration, you want to add a single RunPython operation to it in order to link your records. In this case, it could look something like this:

def link_artists(apps, schema_editor):
    Album = apps.get_model('discography', 'Album')
    Artist = apps.get_model('discography', 'Artist')
    for album in Album.objects.all():
        artist, created = Artist.objects.get_or_create(name=album.artist)
        album.artist_link = artist
        album.save()

Now that your data is transferred to the new field, you could actually be done and leave everything as is, using the new artist_link field for everything. Or, if you want to do a bit of cleanup, you want to create two more migrations.

For your first migration, you will want to delete your original field, artist. For your second migration, rename the new field artist_link to artist.

This is done in multiple steps to ensure that Django recognizes the operations properly.