django-federated-foreign-key

A GenericForeignKey drop-in replacement with ability to point to items in another server. Install with pip:

pip install django-federated-foreign-key

GenericForeignKey

A GenericForeignKey allows pointing to objects of any type. To do this, it uses 2 existing fields on the model:

• A reference to a ContentType entry, usually named content_type and
• An id of the related object, usually named object_id

https://docs.djangoproject.com/en/5.2/ref/contrib/contenttypes/#django.contrib.contenttypes.fields.GenericForeignKey

Difference for Federated Foreign Key

The limitation that FederatedForeignKey address is that ContentType is only designed to reference models that exist within the local system, and thus, GenericForeignKey as well. It is taken as obvious that object_id (the related object id) is the id of the object, in another table, in the same database.

The intent of FederatedForeignKey is to provide the same interface, but expand this to allow referencing objects in different databases.

Usage

Add federated_foreign_key to INSTALLED_APPS and define FEDERATION_PROJECT_NAME in your Django settings:

FEDERATION_PROJECT_NAME = 'project_a'
INSTALLED_APPS = [
    'django.contrib.contenttypes',
    'federated_foreign_key',
    # your apps...
]

Use FederatedForeignKey in place of GenericForeignKey together with GenericContentType.

Example

from federated_foreign_key.fields import FederatedForeignKey
from federated_foreign_key.models import GenericContentType

class Reference(models.Model):
    content_type = models.ForeignKey(GenericContentType, on_delete=models.CASCADE)
    object_id = models.PositiveIntegerField()
    content_object = FederatedForeignKey("content_type", "object_id")

Referencing a remote object

If the referenced item does not exist in the local database, content_object becomes a RemoteObject placeholder. GenericContentType.model_class() returns a stand‑in class that can be used for isinstance checks.

remote_ct = GenericContentType.objects.create(
    project="project_b",
    app_label="testapp",
    model="book",
)
cls = remote_ct.model_class()
ref = Reference.objects.create(content_type=remote_ct, object_id=42)
assert isinstance(ref.content_object, cls)

Development

Install development requirements and run linting and tests:

pip install -r requirements-dev.txt
pip install -e .
pip install -e example_project
ruff check .
flake8
pytest -q
# Run Django's contenttypes tests as shipped in `dj_tests/`
# Use --parallel=1 to avoid multiprocessing issues
python dj_tests/runtests.py contenttypes_tests --parallel=1

Demo

Run the included run_demo.sh script to start two demo servers. The main example_project exposes /books/ which lists local books and books fetched from the remote_project server. SQLite database files are created in each project directory when the script sets the DATABASE_NAME environment variable.

./run_demo.sh

Provided that is running, go visit in your browser the URL:

http://localhost:8000/books/

You should see the example_project showing the reqest, but in the logs from the remote_project, it should show requests coming from the example_project. It shows the names being filled in with values from the remote server.

Remote Model Use Cases

There are 3 cases that this is likely to be used for. This library doesn't provide you with everything you need, just the relationship to store a content_object that may be non-local.

1. Same model, different servers, different data
2. Shared model, different servers, different data
3. Different model, on remote server

Same model, different servers

This is what is illustrated in the demo. Consider that you have 2 book shops, and they both have their own listings of books. A book listing will be duplicate, in the sense that both entries reference the same book. But maybe the two shops have different prices. This helps present a unified list of shopping options for books. Each entry could have its own shopping cart icon, to buy that book from that server, at that cost.

In this case, you would need to create a GenericContentType for the remote model. This can have the same (app_label, model_name) as the local model, which is also a GenericContentType. However, the model is also unique on project name, so this is allowed.

Synchronized tables

In this case we assume a table is sychronized by some other mechanism. This is not in scope of this project, but could be done by something like the DAB resource_registry app.

https://github.com/ansible/django-ansible-base/tree/devel/ansible_base/resource_registry

To avoid setting a specific owner of the table, a value of "shared" for the project name will signify that all servers should treat the item as a local object (within the same DB).

Different model on remote server

Finally, it would likely to useful to reference a model that doesn't exist locally. Say that you were a book shop, but you didn't cary any audiobooks. Perhaps another server has an Audiobook model, and you want to reference audiobooks.

This is a good example of where you would need to create a GenericContentType, where the (app_label, model_name) combination does not exist locally.

Illustration

This shows how FederatedForeignKey works compared to GenericForeignKey.

The object_id values specify the row location in all cases.

graph TD
  %% First scenario: GenericForeignKey
  subgraph GenericForeignKey
    GFKPointerTable[Generic FK Table]
    ContentType[ContentType Table]
    TargetTable[Target Model Table]

    GFKPointerTable -->|content_type_id| ContentType
    GFKPointerTable -->|object_id| TargetTable
  end

  %% Second scenario: FederatedForeignKey
  subgraph FederatedForeignKey
    FFKPointerTable[Federated FK Table]
    GenericContentType[GenericContentType Table]
    LocalTargetTable[Local Target Table]

    FFKPointerTable -->|generic_content_type_id| GenericContentType
    FFKPointerTable -->|object_id| LocalTargetTable
  end

  subgraph External Database
    RemoteTargetTable[Remote Target Table]
  end

  FFKPointerTable -->|object_id| RemoteTargetTable

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