Dear Django community,
in a project, I’m struggling to fix a problem with my database schema and
data migrations.
Here’s what I want to do:
My application used to have a “Person” model. As time went by, it turned
out that we would also need an model for “Organization”. Both should
inherit the (non-abstract) model “Actor”, that holds attributes that are
common between Organizaztions and Persons.
That’s what it should look like in the end:
class Actor(models.Model):
# Actor is new!
name = models.CharField(...)
class Organization(Actor):
# organization is new, too!
# organization attributes ...
class Person(Actor):
biography = models.TextField(...)
To keep existing data, I created an Actor-instance for each person (using
the person’s PK value) and used a data migration to copy the attribute
values that are common for both Organiation and Person to Actor.
Next, I turned the Person model’s PK field into a PK + FK field, thereby
linking it with the Actor table:
class Migration(migrations.Migration):
dependencies = [
('resources', 'xxx'),
]
operations = [
migrations.RenameField(
model_name='person',
old_name='id',
new_name='actor_ptr_id',
)
migrations.AlterField(
model_name='person',
* name='actor_ptr_id',
field=models.OneToOneField(auto_created=True, default=0,
on_delete=django.db.models.deletion.CASCADE,
parent_link=True, primary_key=True, serialize=False,
to='resources.Actor',
db_column='actor_ptr_id'), preserve_default=False*
),
]
My Django app does everything it should – data from both tables (persons
and actor) are join when loading person instances from the database.
Here’s the hard part:
Whenever I run ./manage.py makemigrations now, a new migration is created.
The migration first drops my *actor_ptr_id field and then re-creates it*
(see migration code below). This is quite annoying, because it would break
the links between actor and person table. The point seems to be that Django
doesn't accept creating xxx_ptr_id links manually. Any ideas how to prevent
Django from re-creating this migration over and over again?
class Migration(migrations.Migration):
dependencies = [
('resources', 'xxx'),
]
operations = [
migrations.RemoveField(
model_name='person',
name='actor_ptr_id',
),
migrations.AddField(
model_name='person',
name='actor_ptr',
field=models.OneToOneField(auto_created=True, default=0,
on_delete=django.db.models.deletion.CASCADE, parent_link=True,
primary_key=True, serialize=False, to='resources.actor'),
preserve_default=False,
),
]
Thanks for any ideas and help!
Best regards
Julian
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