framework questions

MongoDB’s aggregation framework processes data through a pipeline of stages. Use the `aggregate` method on collections to define these stages. Common stages include `$match` for filtering, `$group` for grouping documents, and `$sort` for sorting. For example, `db.collection.aggregate([{ $match: { status: 'active' } }, { $group: { _id: '$category', total: { $sum: 1 } } }])` groups documents by category and counts the total.

Django includes a testing framework based on Python's `unittest` module. You write test cases by creating classes that inherit from `django.test.TestCase`. These tests can simulate requests, check responses, and verify the behavior of your application’s components. Run tests using `python manage.py test` to ensure your application works as expected.

To set up a Django REST API, install Django REST framework (DRF) and add it to your `INSTALLED_APPS`. Define serializers to convert your models to JSON and create viewsets or API views to handle HTTP requests. Finally, configure your URL patterns to route API requests to these views using DRF’s routing classes.

Expo is a set of tools and services built around React Native that makes it easier to build, deploy, and iterate on mobile applications. It provides a managed workflow with a range of pre-built libraries and components, and simplifies the process of setting up and maintaining a React Native project. Expo can be used for rapid development, but it also offers an 'ejected' workflow for more advanced use cases where you need full control over native code.