swapper 0.2.0

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.

Tested on Python 2.7 and 3.4, with Django 1.6 and 1.7.

Example Use Case

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 implementations. 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. Swapper extends this approach to apply to any model.

Getting Started

pip install swapper

Usage

Extending the above example, create abstract base classes and default implementations:

# reusableapp/models.py
from django.db import models
from swapper import swappable_setting, get_model_name

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

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

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

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

Then the user 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"

Note: Instead of importing concrete models directly, always use the swapper:

# reusableapp/views.py

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

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

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

Real-World Example

Swapper is used extensively in wq.db, particularly in the vera submodule, which has no less than 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.)