Swapper

Django Swappable Models - No longer only for auth.User!

Swapper is an unofficial API for the undocumented but very powerful Django feature: swappable models. Swapper facilitates implementing arbitrary swappable models in your own reusable apps.

Motivation

Suppose your reusable app has two related tables:

from django.db import models
class Parent(models.Model):
    name = models.TextField()

class Child(models.Model):
    name = models.TextField()
    parent = models.ForeignKey(Parent)

Suppose further that you want to allow the user to subclass either or both of these models and supplement them with their own additional fields. You could use Abstract classes (e.g. BaseParent and BaseChild) for this, but then you would either need to:

1. Avoid putting the foreign key on BaseChild and tell the user they need to do it.
2. Put the foreign key on BaseChild, but make Parent a concrete model that can't be swapped
3. Use swappable models, together with ForeignKeys that read the swappable settings.

This third approach is taken by Django to facilitate swapping the auth.User model. The auth.User swappable code was implemented in a generic way that allows it to be used for any model. Although this capability is currently undocumented while any remaining issues are being sorted out, it has proven to be very stable and useful in our experience.

Swapper is essentially a simple API wrapper around this existing functionality. Note that Swapper is primarily a tool for library authors; users of your reusable app generally should not need to know about Swapper in order to use it. (See the notes on End User Documentation below.)

Real-World Example

Swapper is used extensively in the vera extension to wq.db. vera provides 7 inter-related models, each of which can be swapped out for custom implementations. (Swapper actually started out as part of wq.db.patterns, but was extracted for more general-purpose use.)

Creating a Reusable App

First, make sure you have swapper installed. If you are publishing your reusable app as a Python package, be sure to add swapper to your project's dependencies (e.g. setup.py) to ensure that users of your app don't have errors integrating it.

pip3 install swapper

Extending the above example, you might create two abstract base classes and corresponding default implementations:

# reusableapp/models.py
from django.db import models
import swapper

class BaseParent(models.Model):
    # minimal base implementation ...
    class Meta:
        abstract = True

class Parent(BaseParent):
    # default (swappable) implementation ...
    class Meta:
       swappable = swapper.swappable_setting('reusableapp', 'Parent')

class BaseChild(models.Model):
    parent = models.ForeignKey(swapper.get_model_name('reusableapp', 'Parent'))
    # minimal base implementation ...
    class Meta:
        abstract = True

class Child(BaseChild):
    # default (swappable) implementation ...
    class Meta:
       swappable = swapper.swappable_setting('reusableapp', 'Child')

Loading Swapped Models

In your reusable views and other functions, always use the swapper instead of importing swappable models directly. This is because you might not know whether the user of your app is using your default implementation or their own version.

# reusableapp/views.py

# Might work, might not
# from .models import Parent

import swapper
Parent = swapper.load_model("reusableapp", "Parent")
Child = swapper.load_model("reusableapp", "Child")

def view(request, *args, **kwargs):
    qs = Parent.objects.all()
    # ...

Note: swapper.load_model() is the general equivalent of get_user_model() and subject to the same constraints: e.g. it should not be used until after the model system has fully initialized.

Migration Scripts

Swapper can also be used in Django 1.7+ migration scripts to facilitate dependency ordering and foreign key references. To use this feature in your library, generate a migration script with makemigrations and make the following changes. In general, users of your library should not need to make any similar changes to their own migration scripts. The one exception is if you have multiple levels of swappable models with foreign keys pointing to each other (as in vera).

  # reusableapp/migrations/0001_initial.py

  from django.db import models, migrations
< from django.conf import settings
> import swapper

  class Migration(migrations.Migration):

      dependencies = [
<          migrations.swappable_dependency(settings.REUSABLEAPP_PARENT_MODEL),
>          swapper.dependency('reusableapp', 'Parent')
      ]

      operations = [
          migrations.CreateModel(
              name='Child',
              fields=[
                  ('id', models.AutoField(auto_created=True, serialize=False, primary_key=True, verbose_name='ID')),
              ],
              options={
<                 'swappable': 'REUSABLEAPP_CHILD_MODEL',
>                 'swappable': swapper.swappable_setting('reusableapp', 'Child'),
              },
              bases=(models.Model,),
          ),
          migrations.CreateModel(
              name='Parent',
              fields=[
                  ('id', models.AutoField(auto_created=True, serialize=False, primary_key=True, verbose_name='ID')),
              ],
              options={
<                 'swappable': 'REUSABLEAPP_PARENT_MODEL',
>                 'swappable': swapper.swappable_setting('reusableapp', 'Parent'),
              },
              bases=(models.Model,),
          ),
          migrations.AddField(
              model_name='child',
              name='parent',
<             field=models.ForeignKey(to=settings.REUSABLEAPP_PARENT_MODEL),
>             field=models.ForeignKey(to=swapper.get_model_name('reusableapp', 'Parent')),
              preserve_default=True,
          ),
      ]

End User Documentation

With the above setup, the user of your app can override one or both models in their own app. You might provide them with an example like this:

# myapp/models.py
from reusableapp.models import BaseParent
class Parent(BaseParent):
    # custom implementation ...

Then, tell your users to update their settings to trigger the swap.

# myproject/settings.py
REUSABLEAPP_PARENT_MODEL = "myapp.Parent"

The goal is to make this process just as easy for your end user as swapping the auth.User model is. As with auth.User, there are some important caveats that you may want to inform your users about.

The biggest issue is that your users will probably need to define the swapped model settings before creating any migrations for their implementation of myapp. Due to key assumptions made within Django's migration infrastructure, it is difficult to start out with a default (non-swapped) model and then later to switch to a swapped implementation without doing some migration hacking. This is somewhat awkward - as your users will most likely want to try out your default implementation before deciding to customize it. Unfortunately, there isn't an easy workaround due to how the swappable setting is currently implemented in Django core. This will likely be addressed in future Django versions (see #10 and Django ticket #25313).

API Documentation

Here is the full API for swapper, which you may find useful in creating your reusable app code. End users of your library should generally not need to reference this API.