我正在使用Django和Python 3.7。我无法弄清楚如何编写一个Django查询，其中有一个子查询作为where子句的一部分。这是模特......
class Article(models.Model):
objects = ArticleManager()
title = models.TextField(default='', null=False)
created_on = models.DateTimeField(auto_now_add=True)
class ArticleStat(models.Model):
objects = ArticleStatManager()
article = models.ForeignKey(Article, on_delete=models.CASCADE, related_name='articlestats')
elapsed_time_in_seconds = models.IntegerField(default=0, null=False)
votes = models.FloatField(default=0, null=False)
class StatByHour(models.Model):
index = models.FloatField(default=0)
# this tracks the hour when the article came out
hour_of_day = IntegerField(
null=False,
validators=[
MaxValueValidator(23),
MinValueValidator(0)
]
)
在PostGres中，查询看起来类似于
SELECT *
FROM article a,
articlestat ast
WHERE a.id = ast.article_id
AND ast.votes > 100 * (
SELECT "index"
FROM statbyhour
WHERE hour_of_day = extract(hour from (a.created_on + 1000 * interval '1 second')))
请注意子查询是WHERE子句的一部分
ast.votes > 100 * (select index from statbyhour where hour_of_day = extract(hour from (a.created_on + 1000 * interval '1 second')))
所以我以为我可以这样做......
hour_filter = Func(
Func(
(F("article__created_on") + avg_fp_time_in_seconds * "interval '1 second'"),
function='HOUR FROM'),
function='EXTRACT')
...
votes_criterion2 = Q(votes__gte=F("article__website__stats__total_score") / F(
"article__website__stats__num_articles") * settings.TRENDING_PCT_FLOOR *
StatByHour.objects.get(hour_of_day=hour_filter) * day_of_week_index)
qset = ArticleStat.objects.filter(votes_criterion1 & votes_criterion2,
comments__lte=25)
但这会导致“无法解析关键字'文章'到字段中。选项包括：hour_of_day，id，index，num_articles，total_score”错误。我认为这是因为Django在运行其中的较大查询之前正在调查我的“StatByHour.objects”查询，但我不知道如何重写事物以使子查询同时运行。
编辑：K，将我的子查询移动到实际的“子查询”函数中，并引用我使用OuterRef创建的过滤器...
hour_filter = Func(
Func(
(F("article__created_on") + avg_fp_time_in_seconds * "interval '1 second'"),
function='HOUR FROM'),
function='EXTRACT')
query = StatByHour.objects.get(hour_of_day=OuterRef(hour_filter))
...
votes_criterion2 = Q(votes__gte=F("article__website__stats__total_score") / F(
"article__website__stats__num_articles") * settings.TRENDING_PCT_FLOOR *
Subquery(query) *
day_of_week_index)
qset = ArticleStat.objects.filter(votes_criterion1 & votes_criterion2,
comments__lte=25)
这会导致一个
This queryset contains a reference to an outer query and may only be used in a subquery.
这很奇怪，因为我在子查询中使用它。
编辑＃2：即使根据给出的答案更改查询...
hour_filter = Func(
Func(
(F("article__created_on") + avg_fp_time_in_seconds * "interval '1 second'"),
function='HOUR FROM'),
function='EXTRACT')
query = StatByHour.objects.filter(hour_of_day=OuterRef(hour_filter))[:1]
...
votes_criterion2 = Q(votes__gte=F("article__website__stats__total_score") / F(
"article__website__stats__num_articles") * settings.TRENDING_PCT_FLOOR *
Subquery(query) *
day_of_week_index)
qset = ArticleStat.objects.filter(et_criterion1 & et_criterion2 & et_criterion3,
votes_criterion1 & votes_criterion2,
article__front_page_first_appeared_date__isnull=True,
comments__lte=25)
我仍然得到错误
'Func' object has no attribute 'split'
Subqueries需要是未立即评估的查询，以便可以推迟评估，直到运行外部查询。 get()不适合账单，因为它立即执行并返回一个对象实例而不是Queryset。
然而，用filter代替get，然后取一个[:1]切片应该有效：
StatByHour.objects.filter(hour_of_day=OuterRef('hour_filter')).values('hour_of_day')[:1]
注意OuterRef中的字段引用是字符串文字而不是变量。
此外，子查询需要返回单个列和单个行（因为它们被分配给单个字段），因此values()和上面的切片。
另外，我还没有在Q对象中使用子查询;我不确定它会起作用。您可能必须先在注释中保存子查询输出，然后将其用于过滤器计算。
我发现尽可能多地移动到注释中以澄清正在发生的事情是有帮助的。
您可以使用Extract函数来获取小时数。如果你想要加入更复杂的avg_fp_time_in_seconds东西，你需要定义你自己的Func，我没有尝试复制，因为它应该得到自己的帖子（这可能是'Func' object has no attribute 'split'错误来自的地方）。
# First, add a field for the hour
articles_with_hour = Article.objects.annotate(created_on_hour=ExtractHour('created_on'))
# Set up the subquery, referencing the annotated field
for_this_hour = StatByHour.objects.filter(hour_of_day=OuterRef('created_on_hour'))
# Add the subquery, making sure to slice down to one value
articles_with_hour_index = articles_with_hour.annotate(
index_for_this_hour=Subquery(for_this_hour.values('index')[:1]),
)
# Add the website averages for later calculations
# (note if total_score and num_articles are different field types
# you may need an ExpressionWrapper)
articles_with_avg_website_score = articles_with_hour_index.annotate(
average_article_score_for_website=(
F("website__stats__total_score") / F("website__stats__num_articles")
)
)
# Use the averages to calculate the trending floor for each article
articles_with_trending_floor = articles_with_avg_website_score.annotate(
trending_floor=F('average_article_score_for_website') * settings.TRENDING_PCT_FLOOR,
)
# Set up the criteria, referencing fields that are already annotated on the qs
# ...
votes_gte_trending_floor_for_this_hour_criterion = Q(articlestats__votes__gte=(
F('trending_floor')
* F('index_for_this_hour')
* day_of_week_index # not sure where this comes from?
))
# ...
# Then just filter down (note this is an Article QuerySet, not ArticleStat)
qset = articles_with_trending_floor.filter(
votes_gte_trending_floor_for_this_hour_criterion,
# other criteria
front_page_first_appeared_date__isnull=True,
articlestats__comments__lte=25,
)
这些计算中的许多都可以被浓缩，甚至可以使用多个kwargs在单个annotate调用中完成所有操作，但我认为将其全部放在一起会使其更容易理解。
这肯定像是一个Subquery解决方案。
Django> = 1.11
作为一个警告，我测试了代码，但只是模型，我没有任何数据，所以，这个答案只是努力指出你在正确的方向
# Query that references an outer field from another model, in this case created_on.
# On wich we are performing a lookup in order to "extract" the hour (assuming here)
# a DateTimeField or a TimeField.
stat_by_hour = StatByHour.objects.filter(hour_of_day=OuterRef('created_on__hour'))
# Then filter articles, that have articlestats.votes
# greater than 100 * stat_by_hour.index
result = Article.objects.filter(
articlestats__votes__gt=100 * Subquery(stat_by_hour.values('index')[:1], output_field=FloatField())
)
乍一看，它看起来你可能需要在子查询中执行order_by('index')或order_by('-index')，这样切片[:1]将获得最小值或最大值（取决于您的需要。）
我相信你可以使用它（或非常相似的东西）来实现你想要的。
看看Django queries。我认为您可以通过将SQL基本查询更改为Django提供的内容来解决问题。
如果它不起作用，你可以perform raw SQL queries。
使用通过hour_of_day=ExtractHour(OuterRef('article__created_on') + timedelta(seconds=avg_fp_time_in_seconds))过滤的子查询进行过滤。真正的代码将需要一个额外的ExpressionWrapper，只适用于Django >= 2.1.0：
import datetime
from django.db import models
from django.db.models import F, OuterRef, Subquery, Value
from django.db.models.functions import ExtractHour, Coalesce
from django.db.models.expressions import ExpressionWrapper
relevant_hour_stats = (
StatByHour.objects
.filter(
hour_of_day=ExtractHour(ExpressionWrapper(
OuterRef('article__created_on') # NOTE: `OuterRef()+Expression` works only on Django >= 2.1.0
+
datetime.timedelta(seconds=avg_fp_time_in_seconds),
output_field=models.DateTimeField()
)),
)
.annotate(
votes_threshold=Coalesce(
100.0 * F('index'),
0.0,
output_field=models.FloatField(),
),
)
.order_by('-votes_threshold')
# NOTE: your StatByHour model does not have unique=True on hour_of_day
# field, so there may be several stat for same hour.
# And from your SQL example it's unclear how should they be handled. So I
# assume that "greatest" threshold is needed.
)
article_stats = (
ArticleStat.objects
.all()
.filter(
votes__gt=Coalesce(
Subquery(relevant_hour_stats.values('votes_threshold')[:1]),
Value(0.0),
output_field=models.FloatField(),
),
)
)
附：如果你在github上设置一些“演示项目”会更容易，这样任何人都可以克隆它并在本地检查他们的想法。
P.P.S.此代码经过测试可用，但在不同的型号/字段上：
In [15]: relevant_something = (ModelOne.objects.filter(index=ExtractHour(ExpressionWrapper(OuterRef('due_date') + datetime.timedelta(seconds=1000), output_field=models.DateTimeField()))).annotate(votes_threshold=100*F('indent')).order_by('-votes_threshold'))
In [16]: ts = ModelTwo.objects.all().filter(votes__gt=Subquery(relevant_notes.values('votes_threshold')[:1], output_field=models.IntegerField()))
In [17]: print(ts.query)
SELECT
...
FROM
"some_app_model_two"
WHERE
"some_app_model_two"."votes" > (
SELECT
(100 * U0."indent") AS "votes_threshold"
FROM
"some_app_model_one" U0
WHERE
U0."index" = (
EXTRACT(
'hour'
FROM ("some_app_model_two"."due_date" + 0:16:40)
AT TIME ZONE 'America/Los_Angeles'
)
)
ORDER BY "votes_threshold" DESC
LIMIT 1
)
ORDER BY
"some_app_model_two"."due_date" ASC,
"some_app_model_two"."priority" ASC,
"some_app_model_two"."updated_at" DESC
因此，如果您收到任何错误，请显示您正在运行的ACTUAL代码