> Den 14/07/2015 kl. 23.32 skrev A Lee <anonymous...@gmail.com>:
>
> I'd like to change a ForeignKey field into a ManyToManyField, preserving
> existing data by creating entries in the many-to-many through table. Running
> makemigrations after changing the model class creates a migration that
> executes a RemoveField on the existing ForeignKey field and then an AddField
> on the new ManyToManyField, which destroys any existing ForeignKey data.
>
> I ended up implementing this by creating three migrations:
>
> 1. schema migration: create the new ManyToManyField
> 2. data migration: copy existing ForeignKey data into the ManyToManyField
> 3. schema migration: remove the ForeignKey field and rename the
> ManyToManyField
>
> Is there a simpler way to do this type of schema change?
No. AFAIK, this is the recommended and most robust and flexible way of making
non-trivial changes to a model field definition.
If you have lots of data to migrate, you may need to optimize the data
migration with e.g. raw SQL instead of the naive:
for obj in MyModel.objects.all():
obj.my_m2m.add(obj.my_fk)
Erik
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