id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
20600	Subqueries should retain ORDER BY when using DISTINCT ON	brianglass	anonymous	"When using subqueries in conjunction with DISTINCT ON, ordering should be retained. 

Case in point: if I have a table of chapters in a book with weightings for popularity and I wish to obtain the top weighted chapter per book I might do something like:
{{{
#!python
top_chapters = Chapter.objects.filter(topics__in=['css', 'javascript']).distinct('book_id').order_by('book_id', '-weight', 'id)
}}}
Now I want the chapters to be ordered by weight, so I use top_chapters as a sub-query:
{{{
#!python
chapters = Chapter.objects.filter(id__in=top_chapters).order_by('weight')
}}}
However, the ORDER BY statement is stripped from the subquery so we are not guaranteed to get the top chapters. If the subquery is sliced as in the following, the ordering is retained:
{{{
#!python
count = top_chapters.count()
chapters = Chapter.objects.filter(id__in=top_chapters[:count]).order_by('weight')
}}}

This issue seems related to ticket #12328. In that ticket, order by was intentionally retained if the subquery is sliced."	Bug	closed	Database layer (models, ORM)	1.5	Normal	fixed			Accepted	1	0	0	0	0	0