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Filter collections with Django

The django-oso library can enforce policies over Django models. This allows policies to control access to collections of objects without needing to authorize each object individually.

Installation

The Oso Django integration is available on PyPI and can be installed using pip:

$ pip install django-oso

Usage

The easiest way to prepare your existing Django models for use in a list filtering policy is to have them inherit from django_oso.models.AuthorizedModel(), a thin wrapper around django.models.Model that calls django_oso.auth.authorize_model() under the hood to return Django QuerySets with authorization filters applied.

The policies you write will largely look the same with or without the list filtering feature, and the Oso engine will follow similar evaluation paths.

In the list filtering case, Oso consults the policy to build up a list of conditions that must be met in order for a model to be authorized. These conditions are translated into Django ORM filters and applied to the query before retrieving objects from the database.

Example

Let’s look at an example usage of this library. Our example is a social media app that allows users to view posts. There is a User model and a Post model:

models.py
from django.db import models
from django.contrib.auth.models import AbstractUser

from django_oso.models import AuthorizedModel


class User(AbstractUser):
    is_admin = models.BooleanField(default=False)
    manager = models.ForeignKey(
        "self", null=True, related_name="direct_reports", on_delete=models.CASCADE
    )


class Post(AuthorizedModel):
    contents = models.CharField(max_length=255)
    AccessLevelType = models.TextChoices("AccessLevelType", "public private")
    access_level = models.CharField(
        choices=AccessLevelType.choices, max_length=7, default="private"
    )
    creator = models.ForeignKey(User, on_delete=models.CASCADE)

    class Meta:
        app_label = "app"

We want to enforce the following authorization scheme for posts:

  1. Anyone is allowed to GET any public post.
  2. A user is allowed to GET their own private posts.
  3. A user is allowed to GET private posts made by users they manage (defined through the user.manager relationship).

The corresponding policy looks as follows:

example.polar
# Anyone may view public posts.
allow(_: app::User, "GET", post: app::Post) if
    post.access_level = "public";

# Users may view their own private posts.
allow(user: app::User, "GET", post: app::Post) if
    post.access_level = "private" and
    post.creator = user;

# Users may view private posts created by users who they manage.
allow(user: app::User, "GET", post: app::Post) if
    post.access_level = "private" and
    post.creator in user.direct_reports.all();

Trying it out

If you want to follow along, clone the Oso repository from GitHub and cd into it and then into the docs/examples/data_access/django directory. Then, run make setup to install dependencies (primarily Django and django-oso) and seed the database.

The database now contains a set of four posts made by two users:

manager = User(username="manager")
user = User(username="user", manager=manager)

Post(contents="public user post", access_level="public", creator=user)
Post(contents="private user post", access_level="private", creator=user)
Post(contents="public manager post", access_level="public", creator=manager)
Post(contents="private manager post", access_level="private", creator=manager)

Once everything is set up, run python example/manage.py runserver to start the Django app. We can now use cURL to interact with the application.

A guest user may view public posts:

$ curl localhost:8000/posts
1 - @user - public - public user post
3 - @manager - public - public manager post

A non-manager may view public posts and their own private posts:

$ curl --user user:user localhost:8000/posts
1 - @user - public - public user post
2 - @user - private - private user post
3 - @manager - public - public manager post

A manager may view public posts, their own private posts, and private posts of their direct reports:

$ curl --user manager:manager localhost:8000/posts
1 - @user - public - public user post
2 - @user - private - private user post
3 - @manager - public - public manager post
4 - @manager - private - private manager post

How it works

QuerySets containing authorized models are automatically filtered using constraints derived from the policy.

For example, the above policy has the following rule:

example.polar
allow(user: app::User, "GET", post: app::Post) if
    post.access_level = "private" and
    post.creator = user;

When determining which Post objects User(id=2) is authorized to see, the django-oso adapter converts the constraints on Post expressed in this rule into a Django Q filter:

(AND: ('access_level', 'private'), ('creator__pk', 2))

When composed with filters generated from the other rules, the QuerySet is scoped down to include only authorized objects. The result is the following SQL statement, with the highlighted clause corresponding to the above filter:

SELECT "app_post"."id", "app_post"."contents", "app_post"."access_level", "app_post"."creator_id"
FROM "app_post"
WHERE "app_post"."id" IN (
  SELECT DISTINCT U0."id"
  FROM "app_post" U0
  WHERE (
    U0."access_level" = 'public' OR
    (U0."access_level" = 'private' AND U0."creator_id" = 2)));

Limitations

This feature is still under active development. Not all valid policies are currently supported, but more will be supported as we continue working on this feature. The Django adapter is ready for evaluation and testing. However, we recommend getting in touch with us on Slack before using it in production.

There are some operators and features that do not currently work with the Django adapter when used anywhere in the policy:

  • The cut operator.
  • Rules that rely on ordered execution based on class inheritance.

Some operations cannot be performed on authorized models in rules used with the Django adapter. These operations can still be used on regular Django models or Python objects:

  • Application method calls.
  • Arithmetic operators.

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