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django-auto-prefetching

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Automatic prefetching for Django

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# Django Auto-Prefetching
[![<GeeWee>](https://circleci.com/gh/GeeWee/django-auto-prefetching.svg?style=shield)](https://app.circleci.com/pipelines/github/GeeWee/django-auto-prefetching)

*Never worry about n+1 performance problems again*

This project aims to automatically perform the correct `select_related` and `prefetch_related`
calls for your django-rest-framework code. It does this by inspecting your serializers, seeing what fields
they use, and what models they refer to, and automatically calculating what needs to be prefetched.

## Installation
Installation via <a href="https://pypi.org/project/django-auto-prefetching/">pip</a>:

`pip install django-auto-prefetching`

## AutoPrefetchViewSetMixin
This is a ViewSet mixin you can use, which will automatically prefetch the needed objects from the database, based on the ViewSets `queryset` and `serializer_class`. Under most circumstances this will be all the database optimizations you'll ever need to do:

### Usage
Simply add it after your ModelViewSet class.

```python
from django_auto_prefetching import AutoPrefetchViewSetMixin
from rest_framework.viewsets import ModelViewSet

class BaseModelViewSet(AutoPrefetchViewSetMixin, ModelViewSet):
    queryset = YourModel.objects.all()
    serializer_class = YourModelSerializer
```
It supports all types of relational fields (many to many, one to many, one to one etc.) out of the box.

### Manually calling prefetch
The `AutoPrefetchViewSetMixin` cannot see what objects are being accessed in e.g. a `SerializerMethodField`.
If you use objects in there, you might need to do some additional prefetches.
If you do this and override `get_queryset`, you will have to call `prefetch` manually as the mixin code is never reached.

```python
import django_auto_prefetching
from rest_framework.viewsets import ModelViewSet

class BaseModelViewSet(django_auto_prefetching.AutoPrefetchViewSetMixin, ModelViewSet):
    serializer_class = YourModelSerializer
    
    def get_queryset(self):
        # Simply do the extra select_related / prefetch_related here
        # and leave the mixin to do the rest of the work
        queryset = YourModel.objects.all()
        queryset = queryset.select_related('my_extra_field')
        return django_auto_prefetching.prefetch(queryset, self.serializer_class)
```
You can override `get_prefetchable_queryset` instead of `get_queryset` if you don't want to manually call `django_auto_prefetching.prefetch()`. Example:
```python
import django_auto_prefetching
from rest_framework.viewsets import ModelViewSet

class BaseModelViewSet(django_auto_prefetching.AutoPrefetchViewSetMixin, ModelViewSet):
    serializer_class = YourModelSerializer
    
    def get_prefetchable_queryset(self):
        return YourModel.objects.all()
```
Now `get_queryset()` will call our `get_prefetchable_queryset()` and will add automatic prefetches

## Manually specifying which fields are needed

If you need to explicitly specify some extra fields to be included or excluded, you can also override the following methods on your ViewSet to return a list or a set of fields to prefetch/exclude.

```python
import django_auto_prefetching
from rest_framework.viewsets import ModelViewSet

class BaseModelViewSet(django_auto_prefetching.AutoPrefetchViewSetMixin, ModelViewSet):
    serializer_class = YourModelSerializer

def get_auto_prefetch_excluded_fields(self):
        return {"exclude_this_field", "and_this_field"}
    
    def get_auto_prefetch_extra_select_fields(self):
        return {"select_related_on_this_field"}

def get_auto_prefetch_extra_prefetch_fields(self):
        return {"prefetch_related_on_this_field"}
}
```

## Supported Versions

Python: 3.7, 3.8, 3.10<br>
Django: 3.2, 4.0.4

Pull Requests to support other versions are welcome.

## Maturity
The project is currently being used without issues in a medium-sized Django project (20.000 lines of code).

## Contributing
Contributions are welcome! To get the tests running, do the following:
- Clone the repository.
- If you don't have it, install [pipenv](https://pipenv.pypa.io/en/latest/#install-pipenv-today)
- Install the dependencies with `pipenv sync --dev`
- Activate the virtualenv created by pipenv by writing `pipenv shell`
- Run the tests with `./manage.py test`

django-auto-prefetching

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