ionelmc / Django Prefetch
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Simple and generic model related data prefetch framework for Django solving the "1+N queries" problem that happens when you need related data for your objects.
In most of the cases you'll have forward relations (foreign keys to something) and can use select_related to fetch that data on the same query. However, in some cases you cannot design your models that way and need data from reverse relations (models that have foreign keys to your objects).
Django has prefetch_related_ for this, however, this framework provides greater flexibility than Django's prefetch_related_ queryset method at the cost of writting the mapping and query functions for the data. This has the advantage that you can do things prefetch_related_ cannot (see the latest_book example_ below).
- Free software: BSD license
.. _prefetch_related: https://docs.djangoproject.com/en/dev/ref/models/querysets/#prefetch-related
Installation guide
Install it::
pip install django-prefetch
Use it as your model's default manager (or as a base class if you have custom manager).
Requirements
:OS: Any :Runtime: Python 2.7, 3.3+ or PyPy :Packages: Django>=1.9
Example
Here's a simple example of models and prefetch setup::
from django.db import models
from prefetch import PrefetchManager, Prefetcher
class Author(models.Model):
name = models.CharField(max_length=100)
objects = PrefetchManager(
books = Prefetcher(
filter = lambda ids: Book.objects.filter(author__in=ids),
reverse_mapper = lambda book: [book.author_id],
decorator = lambda author, books=(): setattr(author, 'books', books)
),
latest_book = Prefetcher(
filter = lambda ids: Book.objects.filter(author__in=ids),
reverse_mapper = lambda book: [book.author_id],
decorator = lambda author, books=(): setattr(
author,
'latest_book',
max(books, key=lambda book: book.created)
)
)
)
class Book(models.Model):
class Meta:
get_latest_by = 'created'
name = models.CharField(max_length=100)
created = models.DateTimeField(auto_now_add=True)
author = models.ForeignKey(Author)
Use it like this::
for a in Author.objects.prefetch('books', 'latest_book'):
print a.books
print a.latest_book
Prefetcher arguments
Example models::
class LatestNBooks(Prefetcher):
def __init__(self, count=2):
self.count = count
def filter(self, ids):
return Book.objects.filter(author__in=ids)
def reverse_mapper(self, book):
return [book.author_id]
def decorator(self, author, books=()):
books = sorted(books, key=lambda book: book.created, reverse=True)
setattr(author,
'latest_%s_books' % self.count,
books[:self.count])
class Author(models.Model):
name = models.CharField(max_length=100)
objects = PrefetchManager(
latest_n_books = LatestNBooks
)
Use it like this::
from prefetch import P
for a in Author.objects.prefetch(P('latest_n_books', count=5)):
print a.latest_5_book
.. note::
``P`` is optional and you can only use for prefetch definitions that are Prefetcher subclasses. You can't use it with prefetcher-instance style
definitions like in the first example. Don't worry, if you do, you will get an exception explaining what's wrong.
Other examples
Check out the tests for more examples.
TODO
- Document
collect
option ofPrefetcher
- Create tests covering custom
collect
andmapper