django-expression-filter 0.0.1

A class for Django filters to allow direct filtering on expressions, as you would normally do with annotations, so the annotation does not need to happen.

About

(WIP)

This package provides a class that enables direct filtering on an expression that you’d usually use as an annotation.

Usage

The class AnnotationBypass takes 3 arguments. The first is any valid annotation value, the second is the lookup string you’d use in the filter kwarg name, the third is the value you’d use for the filter.

from expression_filter import AnnotationBypass

filter = AnnotationBypass(Sum('book__page_count'), 'gte', 1000)

But why

Consider the following:

Book.objects.annotate(words_per_page=F('word_count') / F('page_count')).filter(words_per_page__gte=100)

This executes the following query:

SELECT `books_book`.`id`,
       `books_book`.`word_count`,
       `books_book`.`page_count`,
       `books_book`.`word_count` / `books_book`.`page_count` AS `words_per_page`
FROM `books_book`
WHERE `books_book`.`word_count` / `books_book`.`page_count` >= 100

Notice that the expression is duplicated, as using the words_per_page annotation by name in the WHERE clause is not allowed. Usually the annotated value is not needed in the response, and depending on how complex the annotation is or how many objects are returned, this could have a performance impact.

This is better:

from expression_filter import AnnotationBypass

Book.objects.filter(AnnotationBypass(F('word_count') / F('page_count'), 'gte', 100))

SELECT `books_book`.`id`,
       `books_book`.`word_count`,
       `books_book`.`page_count`
FROM `books_book`
WHERE `books_book`.`word_count` / `books_book`.`page_count` >= 100