Permission Rules

Django Rules is a library that provides a lightweight, flexible, and simple way to manage object-level permissions and business logic in Django projects. It allows you to define rules as Python functions and apply them to objects, views, or APIs.
Author

Benedict Thekkel

1. Installation

Install Django Rules via pip:

pip install rules

Add rules to your INSTALLED_APPS in settings.py:

INSTALLED_APPS = [
    # Other apps...
    "rules",
]

2. Key Concepts

  • Rules: Simple Python functions that return True or False based on whether a condition is met.
  • Permissions: Rules tied to specific actions (e.g., add_user, change_post).
  • Object-Level Checks: Rules are ideal for checking permissions at the object level (e.g., “Can this user edit this specific object?”).

3. Defining Rules

Rules are defined using the rules.add_rule function. You can then reuse these rules across your application.

Example: Define a Rule

import rules

# Define a rule to check if the user is the owner of an object
@rules.predicate
def is_owner(user, obj):
    return obj.owner == user

# Add the rule to the rules registry
rules.add_rule("is_owner", is_owner)

You can also define rules using lambda functions:

rules.add_rule("is_admin", lambda user: user.is_staff)

4. Combining Rules

You can combine rules using logical operators (|, &, ~).

# Check if the user is the owner or an admin
rules.add_rule("is_owner_or_admin", is_owner | rules.is_staff)

# Check if the user is the owner and is active
rules.add_rule("is_owner_and_active", is_owner & rules.is_active)

# Negate a rule
rules.add_rule("is_not_owner", ~is_owner)

5. Applying Rules

Object-Level Permission Checks

You can apply rules directly in your views or methods:

from rules.contrib.views import permission_required

# Example usage in a class-based view
@permission_required("is_owner", fn=lambda obj: obj)
def edit_view(request, obj):
    # Your view logic here
    pass

View-Level Permission Checks

Apply permissions to views using decorators:

from rules.contrib.views import permission_required

@permission_required("is_owner_or_admin", fn=lambda obj: obj)
def edit_post(request, post_id):
    post = get_object_or_404(Post, id=post_id)
    return render(request, "edit_post.html", {"post": post})

6. Integration with Django’s Permissions System

Django Rules integrates seamlessly with Django’s built-in permissions system. You can replace or augment the default behavior.

Registering Custom Permissions

Define your custom rules and tie them to permissions:

rules.add_perm("app.change_post", is_owner | rules.is_staff)

In this example: - A user can change a post if they are the owner or a staff member.

7. Rules in Models

You can use rules to enforce permissions at the model level.

Example: Use rules.has_perm in a Model Method

from django.db import models
import rules

class Post(models.Model):
    title = models.CharField(max_length=100)
    content = models.TextField()
    owner = models.ForeignKey('auth.User', on_delete=models.CASCADE)

    def can_edit(self, user):
        return rules.has_perm('app.change_post', user, self)

8. Testing Rules

Since rules are just Python functions, they are straightforward to test.

Example: Testing a Rule

from django.test import TestCase
from myapp.models import Post
from django.contrib.auth.models import User

class RulesTestCase(TestCase):
    def setUp(self):
        self.user = User.objects.create_user(username="user1")
        self.other_user = User.objects.create_user(username="user2")
        self.post = Post.objects.create(title="Test", content="Content", owner=self.user)

    def test_is_owner_rule(self):
        self.assertTrue(is_owner(self.user, self.post))
        self.assertFalse(is_owner(self.other_user, self.post))

9. Debugging Rules

You can debug rules using Django’s logging system:

import logging

logger = logging.getLogger("django.rules")

logger.setLevel(logging.DEBUG)

This can help you track when and how rules are evaluated.

10. Best Practices

  1. Keep Rules Simple: Rules should be lightweight and focused on specific conditions.
  2. Use Predicates: Decorate rules with @rules.predicate to make them composable.
  3. Combine Logically: Use |, &, and ~ to create complex permission rules without redundancy.
  4. Document Rules: Clearly define the purpose of each rule for maintainability.
  5. Test Rules: Test rules independently to ensure they behave as expected.

11. Comparison with Django’s Default Permissions

Feature Django Permissions Django Rules
Predefined Permissions Yes No
Custom Business Logic Limited Fully Customizable
Object-Level Permissions Requires third-party libs Built-in
Ease of Use Straightforward Lightweight, Flexible
Combines Multiple Permissions Requires manual logic Supported using operators

12. Limitations

  • Manual Registration: Rules need to be explicitly registered.
  • Performance: Overuse of complex rules can slow down large-scale applications. Optimize when necessary.
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