django-model-subscription

Table of contents

• Motivation
• Installation
• Usage
• Decorators
• Setup Subscribers using AppConfig.ready
• Setup Subscribers with auto discovery

Motivation

• Using Observer Pattern notify subscribers about changes to a django model.
• Decouple Business logic from Models.save
• Support for bulk actions (Not available using django signals.)
• Use noop subscribers when settings.SUBSCRIPTION_RUN_EXTERNAL is False which prevents having to mock subscribers that call external services in testing, local development environments.
• Show changes to the instance after it has been updated i.e diff's the initial state and the current state.

Installation

$ pip install django-model-subscription

Add model_subscription to your INSTALLED_APPS

INSTALLED_APPS = [
    ...,
    'model_subscription',
    ...
]

Usage

Creating subscribers.

• Using OperationType

import logging
from model_subscription.decorators import subscribe
from model_subscription.constants import OperationType

log = logging.getLogger(__name__)

@subscribe(OperationType.CREATE, TestModel)
def handle_create(instance):
    log.debug('Created {}'.format(instance.name))

• Using create_subscription directly (succinct version).

import logging
from model_subscription.decorators import create_subscription

log = logging.getLogger(__name__)

@create_subscription(TestModel)
def handle_create(instance):
    log.debug('Created {}'.format(instance.name))

Decorators

• subscribe: Explicit (Requires a valid OperationType).

(Create, Update, Delete) operations.

• create_subscription: Subscribes to create operation i.e a new instance.

@create_subscription(TestModel)
def handle_create(instance):
    log.debug('1. Created {}'.format(instance.name))

• update_subscription: Subscribes to updates also includes (changed_data).

@update_subscription(TestModel)
def handle_update(instance, changed_data):
    log.debug('Updated {} {}'.format(instance.name, changed_data))

• delete_subscription: Subscribes to delete operation:

NOTE: The instance.pk is already set to None.

@delete_subscription(TestModel)
def handle_delete(instance):
    log.debug('Deleted {}'.format(instance.name))

(Bulk Create, Bulk Update, Bulk Delete) operations.

• bulk_create_subscription: Subscribe to bulk create operations.

@bulk_create_subscription(TestModel)
def handle_bulk_create(instances):
    for instance in instances:
        log.debug('Bulk Created {}'.format(instance.name))

• bulk_update_subscription: Subscribe to bulk update operations.

@bulk_update_subscription(TestModel)
def handle_bulk_update(instances):
    for instance in instances:
        log.debug('Updated {}'.format(instance.name))

• bulk_delete_subscription: Subscribe to bulk delete operations.

@bulk_delete_subscription(TestModel)
def handle_bulk_delete(instances):
    for instance in instances:
        log.debug('Deleted {}'.format(instance.name))

Setup Subscribers using AppConfig.ready (Recomended).

Update you apps.py

from django.apps import AppConfig


class MyAppConfig(AppConfig):
    name = 'myapp'

    def ready(self):
        from myapp import subscriptions

Setup Subscribers using auto discovery.

By default the settings.SUBSCRIPTION_AUTO_DISCOVER is set to False.

To use auto discovery this is not recommended as it would notify the subscribers wherever the model is used i.e IPython notebook, external scripts.

In your settings.py add

SUBSCRIPTION_AUTO_DISCOVER = True

Setting up the SUBSCRIPTION_MODULE

NOTE: This is only required when SUBSCRIPTION_AUTO_DISCOVER = True

SUBSCRIPTION_MODULE  = 'subscription' 

Resources

• https://python-3-patterns-idioms-test.readthedocs.io/en/latest/Observer.html
• https://refactoring.guru/design-patterns/observer
• https://hackernoon.com/observer-vs-pub-sub-pattern-50d3b27f838c

TODO's

• Supporting field level subscriptions.
• Support class based subscribers which implements __call__
• Extend to include custom OperationType.
• Add support for using a single class to manage multiple actions i.e MyClass.update, MyClass.create.