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django-auto-prefetch-1.5.1
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مشاهده بیشترتوضیحات
Automatically prefetch foreign key values as needed.
| ویژگی | مقدار |
|---|---|
| سیستم عامل | - |
| نام فایل | django-auto-prefetch-1.5.1 |
| نام | django-auto-prefetch |
| نسخه کتابخانه | 1.5.1 |
| نگهدارنده | [] |
| ایمیل نگهدارنده | [] |
| نویسنده | Gordon Wrigley |
| ایمیل نویسنده | gordon.wrigley@gmail.com |
| آدرس صفحه اصلی | https://github.com/tolomea/django-auto-prefetch |
| آدرس اینترنتی | https://pypi.org/project/django-auto-prefetch/ |
| مجوز | BSD-3-Clause |
django-auto-prefetch
====================
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Automatically prefetch foreign key values as needed.
Purpose
-------
When accessing a ``ForeignKey`` or ``OneToOneField`` (including in reverse) on
a model instance, if the field’s value has not yet been loaded then
auto-prefetch will prefetch the field for all model instances loaded by
the same ``QuerySet`` as the current model instance. This is enabled at the
model level and totally automatic and transparent for users of the
model.
Requirements
------------
Python 3.7 to 3.11 supported.
Django 3.2 to 4.2 supported.
Usage
-----
1. Install with ``python -m pip install django-auto-prefetch``.
2. Change all these imports from ``django.db.models`` to ``auto_prefetch``:
* ``ForeignKey``
* ``Manager``
* ``Model`` - including inheriting ``Meta`` from ``auto_prefetch.Model.Meta``
* ``OneToOneField``
* ``QuerySet``
If you use custom subclasses of any of these classes, you should be able to swap for the ``auto_prefetch`` versions in your subclasses’ bases.
For example, if you had:
.. code:: python
from django.db import models
class Book(models.Model):
author = models.ForeignKey("Author", on_delete=models.CASCADE)
class Meta:
verbose_name = "Book"
…swap to:
.. code:: python
import auto_prefetch
from django.db import models
class Book(auto_prefetch.Model):
author = auto_prefetch.ForeignKey("Author", on_delete=models.CASCADE)
class Meta(auto_prefetch.Model.Meta):
verbose_name = "Book"
3. Run ``python manage.py makemigrations`` to generate migrations for all the models you modified.
These migrations will set the |Meta.base_manager_name option|__ to ``prefetch_manager`` for every model that you’ve converted.
This change ensures that auto-prefetching happens on related managers.
Such migrations do not change anything in the database.
.. |Meta.base_manager_name option| replace:: ``Meta.base_manager_name`` option
__ https://docs.djangoproject.com/en/stable/ref/models/options/#base-manager-name
(If you instead set ``Meta.base_manager_name`` on your models, make sure it inherits from ``auto_prefetch.Manager``.)
Background and Rationale
------------------------
Currently when accessing an uncached foreign key field, Django will
automatically fetch the missing value from the database. When this
occurs in a loop it creates 1+N query problems. Consider the following
snippet:
.. code:: python
for choice in Choice.objects.all():
print(choice.question.question_text, ":", choice.choice_text)
This will do one query for the choices and then one query per choice to
get that choice’s question.
This behavior can be avoided with correct application of
``prefetch_related()`` like this:
.. code:: python
for choice in Choice.objects.prefetch_related("question"):
print(choice.question.question_text, ":", choice.choice_text)
This has several usability issues, notably:
* Less experienced users are generally not aware that it’s necessary.
* Cosmetic seeming changes to things like templates can change the fields that
should be prefetched.
* Related to that, the code that requires the ``prefetch_related()`` (e.g. the
template) may be quite removed from where the ``prefetch_related()`` needs to
be applied (e.g. the view).
* Subsequently finding where ``prefetch_related()`` / ``select_related()``
calls are missing is non-trivial and needs to be done on an ongoing
basis.
* Excess entries in ``prefetch_related()`` calls are even harder
to find and result in unnecessary database queries.
* It is very difficult for libraries like the admin and Django Rest Framework
to automatically generate correct ``prefetch_related()`` clauses.
On the first iteration of the loop in the example above, when we first
access a choice’s question field, instead of fetching the question for
just that choice, auto-prefetch will speculatively fetch the questions
for all the choices returned by the ``QuerySet``. This change results in
the first snippet having the same database behavior as the second while
reducing or eliminating all of the noted usability issues.
Some important points:
* ``ManyToManyField``\s are not changed at all.
* Because these are ``ForeignKey`` and ``OneToOneField``\s, the
generated queries can’t have more result rows than the original query
and may have less. This eliminates any concern about a multiplicative
query size explosion.
* This feature will never result in more database queries as a prefetch will
only be issued where the ORM was already going to fetch a single related
object.
* Because it is triggered by fetching missing related objects it will not at
all change the DB behavior of code which is fully covered by
``prefetch_related()`` and/or ``select_related()`` calls.
* This will inherently chain across relations like ``choice.question.author``.
The conditions above still hold under such chaining.
* In some rare situations it may result in larger data transfer between the
database and Django (see below).
An example of that last point is:
.. code:: python
qs = Choice.objects.all()
list(qs)[0].question
Such examples generally seem to be rarer and more likely to be visible
during code inspection (vs ``{{ choice.question }}`` in a template). And
larger queries are usually a better failure mode than producing hundreds
of queries. For this to actually produce inferior behavior in practice
you need to:
* fetch a large number of choices
* filter out basically all of them
* ...in a way that prevents garbage collection of the unfiltered ones
If any of those aren’t true then automatic prefetching will still
produce equivalent or better database behavior than without.
See Also
--------
* The phabricator guide to the N+1 queries problem:
https://secure.phabricator.com/book/phabcontrib/article/n_plus_one/
* The django-developers mailing list discussion of adding the feature
to core Django:
https://groups.google.com/forum/m/#!topic/django-developers/EplZGj-ejvg
* The nplus package, useful for detecting the N+1 queries problem in
your application (but not solving it):
https://pypi.org/project/nplusone/
P.S.
----
If you have concerns go look at the code, it’s all in
`auto_prefetch/__init__.py <https://github.com/tolomea/django-auto-prefetch/blob/main/src/auto_prefetch/__init__.py>`__
and is fairly short.
نیازمندی
| مقدار | نام |
|---|---|
| >=3.2 | Django |
زبان مورد نیاز
| مقدار | نام |
|---|---|
| >=3.7 | Python |
نحوه نصب
نصب پکیج whl django-auto-prefetch-1.5.1:
pip install django-auto-prefetch-1.5.1.whl
نصب پکیج tar.gz django-auto-prefetch-1.5.1:
pip install django-auto-prefetch-1.5.1.tar.gz