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.
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 1.4 has prefetch_related for this, however, this framework provides greater flexibility than Django 1.4’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 bellow).
pip install django-prefetch
Use it as your model’s default manager (or as a base class if you have custom manager).
|Runtime:||Python 2.6, 2.7, 3.3+ or PyPy|
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
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
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.
Check out the tests for more examples.
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