[英]How to annotate Count with a condition in a Django queryset
使用Django ORM,可以執行類似queryset.objects.annotate(Count('queryset_objects', gte=VALUE))
。 抓住我的漂移?
這是一個用於說明可能答案的簡單示例:
在Django網站中,內容創建者提交文章,並且普通用戶查看(即閱讀)所述文章。 文章可以發表(即可供所有人閱讀),也可以草稿模式。 描述這些要求的模型是:
class Article(models.Model):
author = models.ForeignKey(User)
published = models.BooleanField(default=False)
class Readership(models.Model):
reader = models.ForeignKey(User)
which_article = models.ForeignKey(Article)
what_time = models.DateTimeField(auto_now_add=True)
我的問題是:如何獲得所有發表的文章,按照過去30分鍾的獨特讀者排序? 即我想要計算每個發表的文章在過去半小時內獲得的不同(獨特)視圖的數量,然后生成按這些不同視圖排序的文章列表。
我試過了:
date = datetime.now()-timedelta(minutes=30)
articles = Article.objects.filter(published=True).extra(select = {
"views" : """
SELECT COUNT(*)
FROM myapp_readership
JOIN myapp_article on myapp_readership.which_article_id = myapp_article.id
WHERE myapp_readership.reader_id = myapp_user.id
AND myapp_readership.what_time > %s """ % date,
}).order_by("-views")
這引發了錯誤: 語法錯誤在“01”或附近 (其中“01”是額外的日期時間對象)。 繼續下去並不多。
使用條件聚合 :
from django.db.models import Count, Case, When, IntegerField
Article.objects.annotate(
numviews=Count(Case(
When(readership__what_time__lt=treshold, then=1),
output_field=IntegerField(),
))
)
說明:通過您的文章的正常查詢將使用numviews
字段進行注釋。 該字段將被構造為CASE / WHEN表達式,由Count包裝,對於讀者匹配標准將返回1,對於不匹配標准的讀者將返回NULL
。 Count將忽略空值並僅計算值。
您將在最近未查看的文章上獲得零,並且您可以使用該numviews
字段進行排序和過濾。
PostgreSQL背后的查詢將是:
SELECT
"app_article"."id",
"app_article"."author",
"app_article"."published",
COUNT(
CASE WHEN "app_readership"."what_time" < 2015-11-18 11:04:00.000000+01:00 THEN 1
ELSE NULL END
) as "numviews"
FROM "app_article" LEFT OUTER JOIN "app_readership"
ON ("app_article"."id" = "app_readership"."which_article_id")
GROUP BY "app_article"."id", "app_article"."author", "app_article"."published"
如果我們只想跟蹤唯一的查詢,我們可以在Count
添加區別,並使我們的When
子句返回值,我們希望區別開來。
from django.db.models import Count, Case, When, CharField, F
Article.objects.annotate(
numviews=Count(Case(
When(readership__what_time__lt=treshold, then=F('readership__reader')), # it can be also `readership__reader_id`, it doesn't matter
output_field=CharField(),
), distinct=True)
)
這會產生:
SELECT
"app_article"."id",
"app_article"."author",
"app_article"."published",
COUNT(
DISTINCT CASE WHEN "app_readership"."what_time" < 2015-11-18 11:04:00.000000+01:00 THEN "app_readership"."reader_id"
ELSE NULL END
) as "numviews"
FROM "app_article" LEFT OUTER JOIN "app_readership"
ON ("app_article"."id" = "app_readership"."which_article_id")
GROUP BY "app_article"."id", "app_article"."author", "app_article"."published"
您可以使用raw
來執行由較新版本的django創建的SQL語句。 顯然沒有簡單和優化的方法來查詢數據而不使用raw
數據(即使有extra
注入所需的JOIN
子句也存在一些問題)。
Articles.objects.raw('SELECT'
' "app_article"."id",'
' "app_article"."author",'
' "app_article"."published",'
' COUNT('
' DISTINCT CASE WHEN "app_readership"."what_time" < 2015-11-18 11:04:00.000000+01:00 THEN "app_readership"."reader_id"'
' ELSE NULL END'
' ) as "numviews"'
'FROM "app_article" LEFT OUTER JOIN "app_readership"'
' ON ("app_article"."id" = "app_readership"."which_article_id")'
'GROUP BY "app_article"."id", "app_article"."author", "app_article"."published"')
對於django> = 2.0,您可以在聚合函數中使用帶有filter
參數的條件聚合 :
from datetime import timedelta
from django.utils import timezone
from django.db.models import Count, Q # need import
Article.objects.annotate(
numviews=Count(
'readership__reader__id',
filter=Q(readership__what_time__gt=timezone.now() - timedelta(minutes=30)),
distinct=True
)
)
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