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swapper 0.2.2

The unofficial Django swappable models API.

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.

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.)

Getting Started

pip3 install swapper

Usage

Extending the above example, create abstract base classes and 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')

User Customization

With the above setup, the user of your app can override one or both models in their own app:

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

The user then specifies the appropriate setting to trigger the swap:

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

Loading Swapped Models

In your views and other functions, always use the swapper instead of importing swappable models directly.

# 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() <https://docs.djangoproject.com/en/1.7/topics/auth/customizing/#referencing-the-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, generate a migration script with makemigrations and make the following changes:

  # 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,
          ),
      ]

API Documentation

function purpose
swappable_setting(app_label, model) Generates a swappable setting name for the provided model (e.g. "REUSABLEAPP_PARENT_MODEL")
is_swapped(app_label, model) Determines whether or not a given model has been swapped. (Returns the model name if swapped, otherwise False)
get_model_name(app_label, model) Gets the name of the model the swappable model has been swapped for (or the name of the original model if not swapped.)
get_model_names(app_label, models) Match a list of model names to their swapped versions. All of the models should be from the same app (though their swapped versions need not be).
load_model(app_label, model, required=True) Load the swapped model class for a swappable model (or the original model if it hasn’t been swapped). If your code can function without the specified model, set required = False.
dependency(app_label, model) Generate a dependency tuple for use in Django 1.7+ migrations.
set_app_prefix(app_label, prefix) Set a custom prefix for swappable settings (the default is the upper case app_label). Used in wq.db to make all of the swappable settings start with "WQ" (e.g. WQ_FILE_MODEL instead of FILES_FILE_MODEL). This should be set at the top of your models.py.
join(app_label, model), split(model) Utilities for splitting and joining "app.Model" strings and ("app", "Model") tuples.
 
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swapper-0.2.2.tar.gz (md5) Source 2015-06-15 5KB
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