[英]Django: Using Annotate, Count and Distinct on a Queryset
這是我的數據庫查詢:
results = Attachments.objects.filter(currency='current').annotate(num_attachments=Count('article_id')).order_by("num_attachments").distinct('article_id')
查詢細分如下(據我所知):
我在PostgreSQL上運行它,所以根據Django 文檔 ,我可以根據字段運行distinct()。
執行查詢時沒有錯誤,但是當我嘗試迭代甚至打印結果時,Django調試會拋出以下錯誤:
NotImplementedError at /function/
annotate() + distinct(fields) not implemented.
交互式提示中更詳細的回溯是:
File "<console>", line 1, in <module>
File "/Users/Pat/.virtualenvs/envsp/lib/python2.7/site-packages/django/db/models/query.py", line 118, in _result_iter
self._fill_cache()
File "/Users/Pat/.virtualenvs/envsp/lib/python2.7/site-packages/django/db/models/query.py", line 875, in _fill_cache
self._result_cache.append(self._iter.next())
File "/Users/Pat/.virtualenvs/envsp/lib/python2.7/site-packages/django/db/models/query.py", line 291, in iterator
for row in compiler.results_iter():
File "/Users/Pat/.virtualenvs/envsp/lib/python2.7/site-packages/django/db/models/sql/compiler.py", line 763, in results_iter
for rows in self.execute_sql(MULTI):
File "/Users/Pat/.virtualenvs/envsp/lib/python2.7/site-packages/django/db/models/sql/compiler.py", line 808, in execute_sql
sql, params = self.as_sql()
File "/Users/Pat/.virtualenvs/envsp/lib/python2.7/site-packages/django/db/models/sql/compiler.py", line 107, in as_sql
"annotate() + distinct(fields) not implemented.")
NotImplementedError: annotate() + distinct(fields) not implemented.
誰知道這里發生了什么?
解決方法是使用values('distinct_fieldname')
因為這將使最終的SQL語句在該字段上執行GROUP BY
(您可以添加多個字段名),這基本上是相同的。
例如,如果您想知道給定'filename'
存在多少篇文章,您可以這樣做:
results = Attachments.objects.filter(currency='current').values('filename').annotate(num_attachments=Count('article_id')).order_by("num_attachments")
我找到了另一種方法,如何克服這個問題 - 使用子查詢:
distinct_articles = Attachments.objects.distinct('article_id')
results = Attachments.objects.filter(currency='current').annotate(num_attachments=Count('article_id')).order_by("num_attachments").filter(id__in=distinct_articles)
這實際上被評估為Django中的一個數據庫查詢。
聲明:本站的技術帖子網頁,遵循CC BY-SA 4.0協議,如果您需要轉載,請注明本站網址或者原文地址。任何問題請咨詢:yoyou2525@163.com.