Queryset transform

django_queryset_transform

Allows you to register a transforming map function with a Django QuerySet that will be executed only when the QuerySet itself has been evaluated.

This allows you to build optimisations like “fetch all tags for these 10 rows” while still benefiting from Django’s lazy QuerySet evaluation.

For example:

def lookup_tags(item_qs):
    item_pks = [item.pk for item in item_qs]
    m2mfield = Item._meta.get_field('tags')[0]
    tags_for_item = Tag.objects.filter(
        item__in = item_pks
    ).extra(select = {
        'item_id': '%s.%s' % (
            m2mfield.m2m_db_table(), m2mfield.m2m_column_name()
        )
    })
    tag_dict = {}
    for tag in tags_for_item:
        tag_dict.setdefault(tag.item_id, []).append(tag)
    for item in item_qs:
        item.fetched_tags = tag_dict.get(item.pk, [])

qs = Item.objects.filter(name__contains = 'e').transform(lookup_tags)

for item in qs:
    print(item, item.fetched_tags)

Prints:

Winter comes to Ogglesbrook [<sledging>, <snow>, <winter>, <skating>]
Summer now [<skating>, <sunny>]

But only executes two SQL queries - one to fetch the items, and one to fetch ALL of the tags for those items.

Since the transformer function can transform an evaluated QuerySet, it doesn’t need to make extra database calls at all - it should work for things like looking up additional data from a cache.multi_get() as well.

Originally inspired by http://github.com/lilspikey/django-batch-select/

LICENSE

Copyright (c) 2010, Simon Willison. All rights reserved.

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

  1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
  2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.
  3. Neither the name of Django nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission.

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class towel.queryset_transform.TransformQuerySet(*args, **kwargs)