Python Django预取与大型数据集相关

我现在与django的预回迁相关的问题。 举个例子，让我们想象一下这些模型

from django.db import models

class Client(models.Model):
    name = models.CharField(max_length=255)

class Purchase(models.Model):
    client = models.ForeignKey('Client')

让我们想象一下，我们有几个客户，大约200个，但他们购买了很多，所以我们有数百万的购买

如果我必须创建一个显示所有客户和每个客户的购买数量的网页，我必须这样写

from django.db.models import Prefetch
from .models import Purchase, Client

purchases = Purchase.objects.all()
clients = Client.prefetch_related(Prefetch('purchase_set', queryset=purchases))

这里的问题是，我将查询big purchases数据库，而该查询可能需要一分钟以上的时间，或者更糟糕的是，在服务器上创建一个MemoryError

因此，我尝试只选择一批具有

 purchases = Purchase.objects.all()[:9]

但正如我们所料，Django不太喜欢它，并推出了这种例外

Traceback (most recent call last):
  File "project/venv/lib/python3.6/site-packages/django/core/handlers/base.py",
 line 149, in get_response
    response = self.process_exception_by_middleware(e, request)
  File "project/venv/lib/python3.6/site-packages/django/core/handlers/base.py",
 line 147, in get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "project/venv/lib/python3.6/site-packages/django/views/generic/base.py",
 line 68, in view
    return self.dispatch(request, *args, **kwargs)
  File "project/venv/lib/python3.6/site-packages/django/utils/decorators.py", l
ine 67, in _wrapper
    return bound_func(*args, **kwargs)
  File "project/venv/lib/python3.6/site-packages/django/views/decorators/cache.
py", line 57, in _wrapped_view_func
    response = view_func(request, *args, **kwargs)
  File "project/venv/lib/python3.6/site-packages/django/utils/decorators.py", l
ine 63, in bound_func
    return func.__get__(self, type(self))(*args2, **kwargs2)
****************** login decorators, views, ... 
  File "project/***.py", line ***, in ***
    for client in clients:
  File "project/venv/lib/python3.6/site-packages/django/db/models/query.py", li
ne 258, in __iter__
    self._fetch_all()
  File "project/venv/lib/python3.6/site-packages/django/db/models/query.py", li
ne 1076, in _fetch_all
    self._prefetch_related_objects()
  File "project/venv/lib/python3.6/site-packages/django/db/models/query.py", li
ne 656, in _prefetch_related_objects
    prefetch_related_objects(self._result_cache, self._prefetch_related_lookups)
  File "project/venv/lib/python3.6/site-packages/django/db/models/query.py", li
ne 1457, in prefetch_related_objects
    obj_list, additional_lookups = prefetch_one_level(obj_list, prefetcher, lookup, level)
  File "project/venv/lib/python3.6/site-packages/django/db/models/query.py", li
ne 1556, in prefetch_one_level
    prefetcher.get_prefetch_queryset(instances, lookup.get_current_queryset(level)))
  File "project/venv/lib/python3.6/site-packages/django/db/models/fields/relate
d_descriptors.py", line 539, in get_prefetch_queryset
    queryset = queryset.filter(**query)
  File "project/venv/lib/python3.6/site-packages/django/db/models/query.py", li
ne 790, in filter
    return self._filter_or_exclude(False, *args, **kwargs)
  File "project/venv/lib/python3.6/site-packages/django/db/models/query.py", li
ne 802, in _filter_or_exclude
    "Cannot filter a query once a slice has been taken."
AssertionError: Cannot filter a query once a slice has been taken.

所以现在，我没有真正的解决办法。我正在研究django/db/models/query.py:258中的_iter____）函数是如何构建的，以尝试创建一个具有相同行为的函数，但需要在预取中设置一个有限的集才能对其进行分页，并以更并行的方式进行操作

有没有什么“好办法”来进行这种查询

让我们想象一下，我们有几个客户，大约200个，但他们购买 很多，所以我们有数以百万计的购买

如果我必须创建一个显示所有客户端和 每个客户的购买数量

我将把你的问题解释为想要这个功能。您是否尝试过：

from django.db.models import Count
clients = Client.objects.annotate(num_purchases=Count('purchase'))
clients[0].num_purchases

如果您希望排序并获得最高的采购客户，您还可以执行以下操作：

clients = Client.objects.annotate(num_purchases=Count('purchase')).order_by('-num_purchases')[:5]

---

有关更多功能，请参阅。

非常感谢您，这正是我想要的，很抱歉，我没有充分阅读手册^”