A library designed to workaround some drawbacks with Django's union, intersection, and difference operations.
django-delayed-union is library designed to workaround some drawbacks with Django’s union, intersection, and difference operations. In particular, once one of these operations is performed, certain methods on the queryset will silently not work:
>>> qs = User.objects.filter(id=1) >>> unioned_qs = qs.union(qs) >>> should_be_empty_qs = unioned_qs.exclude(id=1) >>> user, = list(should_be_empty_qs); user.id 1
In order to work around this, django-delayed-union provides wrappers around a collection of querysets. These wrappers implement a similar interface to QuerySet, and delay performing the union, intersection, or difference operations until they are needed:
>>> from django_delayed_union import DelayedUnionQuerySet >>> qs = User.objects.filter(id=1) >>> unioned_qs = DelayedUnionQuerySet(qs, qs) >>> empty_qs = unioned_qs.exclude(id=1) >>> list(empty_qs) 
Operations which would typically return a new QuerySet instead return a new DelayedQuerySet with the operation applied to its collection of querysets.
One example of where this code has been useful with is when the the MySQL query planner has chosen an inefficient query plan for the queryset of a Django REST Framework view which used an OR condition. By using DelayedUnionQuerySet, subclasses could perform additional filters on the queryset while still maintaining the efficient query plan.
- Free software: BSD 3-Clause License
pip install django-delayed-union
To run the all tests run:
- Added query property to delayed querysets.
- Fixed bug with count() and select_related() in MySQL
- Added tests for Django 3.0
- Added tests for Django 2.2
- Added support for nested unions and intersections
- Cached the queryset generated after applying the delayed operation.
- First release on PyPI.
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