postgresql Django：快速检索 manyToMany 字段的 ID
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Django: Retrieving IDs of manyToMany fields quickly
提问by Noah Gilmore
I have the following model schema in Django (with Postgres).
我在 Django 中有以下模型模式（使用 Postgres）。
class A(Models.model):
related = models.ManyToManyField("self", null=True)
Given a QuerySet of A, I would like to return a dictionary mapping each instance of Ain the QuerySet to a list of ids of its relatedinstances as quickly as possible.
给定 A 的 QuerySet，我想尽快返回一个字典A，将 QuerySet 中的每个实例映射到id其related实例的s列表。
I can surely iterate through each A and query the related field, but is there a more optimal way?
我当然可以遍历每个 A 并查询相关字段，但是有没有更优化的方法？
回答by Luan Fonseca
According you have Three instances. You can use the values_listmethod to retrieve just the results and from this result get just the ID's of their relatedinstances.
I use the pkfield to be my filter because i don't know your scheme, but you can use anything, just must be a QuerySet.
根据你有三个实例。您可以使用该values_list方法仅检索结果，并从该结果中仅获取其related实例的 ID 。我使用该pk字段作为我的过滤器，因为我不知道您的方案，但是您可以使用任何内容，但必须是QuerySet.
>>> result = A.objects.filter(pk=1)
>>> result.values('related__id')
[{'id': 2}, {'id': 3}]
>>> result.values_list('related__id')
[(2,), (3,)]
>>> result.values_list('related__id', flat=True)
[2, 3]
回答by DylanYoung
You can get pretty close like this:
你可以像这样接近：
qs = A.objects.prefetch_related(Prefetch(
'related',
queryset=A.objects.only('pk'),
to_attr='related_insts')).in_bulk(my_list_of_pks)
This will give a mapping from pks of the current object to the instance itself, so you can iterate through as follows:
这将给出从当前对象的 pks 到实例本身的映射，因此您可以按如下方式进行迭代：
for pk, inst in qs.iteritems():
related_ids = (related.pk for related in inst.related_insts)
Or given an instance, you can do a fast lookup like so:
或者给定一个实例，您可以像这样进行快速查找：
related_ids = (related.pk for related in qs[instance.pk]).
This method maps the instance ids to the related ids (indirectly) since you specifically requested a dictionary. If you aren't doing lookups, you may want the following instead:
此方法将实例 ID 映射到相关 ID（间接），因为您特别请求了字典。如果您不进行查找，则可能需要以下内容：
qs = A.objects.prefetch_related(Prefetch(
'related',
queryset=A.objects.only('pk'),
to_attr='related_insts')).filter(pk__in=my_list_of_pks)
for inst in qs:
related_ids = (related.pk for related in inst.related_insts)
You may take note of the use of onlyto only pull the pks from the db. There is an open ticketto allow the use of valuesand (I presume) values_listin Prefetch queries. This would allow you to do the following.
您可能会注意到使用only仅从数据库中提取 pks。有一个开放票允许在预取查询中使用values和（我认为）values_list。这将允许您执行以下操作。
qs = A.objects.prefetch_related(Prefetch(
'related',
queryset=A.objects.values_list('pk', flat=True),
to_attr='related_ids')).filter(pk__in=my_list_of_pks)
for inst in qs:
related_ids = inst.related_ids
You could of course optimize further, for example by using qs.only('related_insts')on the primary queryset, but make sure you aren't doing anything with these instances-- they're essentially just expensive containers to hold your related_ids.
您当然可以进一步优化，例如通过qs.only('related_insts')在主查询集上使用，但请确保您没有对这些实例做任何事情——它们本质上只是用于保存您的 related_ids 的昂贵容器。
I believe this is the best that's available for now (without custom queries). To get to exactly what you want, two things are needed:
我相信这是目前最好的（没有自定义查询）。为了得到你想要的东西，需要做两件事：
- The feature above is implemented
values_listis made to work with Prefetchto_attrlike it does for annotations.
- 上面的功能实现了
values_list与 Prefetch 一起使用，to_attr就像注释一样。
With these two things in place (and continuing the above example) you could do the following to get exactly what you requested:
有了这两件事（并继续上面的示例），您可以执行以下操作以准确获得您的要求：
d = qs.values_list('related_ids', flat=True).in_bulk()
for pk, related_pks in d:
print 'Containing Objects %s' % pk
print 'Related objects %s' % related_pks
# And lookups
print 'Object %d has related objects %s' % (20, d[20])
I've left off some details explaining things, but it should be pretty clear from the documentation. If you need any clarification, don't hesitate!
我已经省略了一些解释事情的细节，但从文档中应该很清楚。如果您需要任何说明，请不要犹豫！
