Question

Let’s supposed I created two models:

class Car(models.Model):
    name =  models.CharField(max_length=50)
    size =  models.IntegerField()


class Manufacturer(models.Model):
    name =  models.CharField(max_length=50)
    country =  models.CharField(max_length=50)
    car = models.ManyToManyField(Car)

I added entries to both models, then I realized that each Car was only related to a unique Manufacturer. So, I should convert my ManyToManyField to a ForeignKey:

class Car(models.Model):
    name =  models.CharField(max_length=50)
    size =  models.IntegerField()
    manufacturer = models.ForeignKey(Manufacturer)

class Manufacturer(models.Model):
    name =  models.CharField(max_length=50)
    country =  models.CharField(max_length=50)

How can I do that without losing my entries? I tried to look in South documentation but I did not found this way of conversion…

Answer 1

This is nontrivial, I think you will need three migrations:

1. Add the ForeignKey.
2. Convert the ManyToMany to ForeignKey (using the forwards method).
3. Remove the ManyToMany.

You could possibly merge 1 and 2 or 2 and 3 together, but I wouldn’t recommend it.
Additionally, you should also implement a backwards method for 2.

An example for 2.’s forwards would be:

class Migration(SchemaMigration):
    def forwards(self, orm):
        for manufacturer in orm.Manufacturer.objects.all():
             for car in manufacturer.car.all():
                  car.manufacturer = manufacturer
                  car.save()

Please note that:

• There is no backwards method here yet.
• This needs to be tested extensively: a migration is something you should be extra careful about.
• In case a car has two manufacturers, the last one will be kept.
• This is very inefficient, we do a Query per car per manufacturer!

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You will also need to update the code that uses those relationships in step 2. / 3.