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
32116	QuerySet.order_by() crashes on union() queryset with a single non-empty query.	Hannes Ljungberg	hannes ljungberg	"I noticed this error while implementing a dynamic union like this:
{{{
union = reduce(
    lambda qs1, qs2: qs1.union(qs2),
    querysets,
    queryset.none(),
)
}}}

If `len(querysets) == 1`  it will result in a pretty weird query throwing a database error when both querysets are ordered, another example recreating this in the test suite:

{{{
qs1 = Number.objects.all().order_by(‘pk’)
qs2 = Number.objects.none().union(qs1).order_by(‘pk’)
}}}

Executing `qs2` result in the following query:

{{{
(SELECT ""queries_number"".""id"", ""queries_number"".""num"", ""queries_number"".""other_num"", ""queries_number"".""another_num"" FROM ""queries_number"" ORDER BY ""queries_number"".""id"" ASC) ORDER BY (1) ASC
}}}

Result in the following error on PostgresSQL:

{{{
psycopg2.errors.SyntaxError: multiple ORDER BY clauses not allowed
LINE 1: ...umber"" ORDER BY ""queries_number"".""id"" DESC) ORDER BY (1) ASC
}}}

And sqlite:

{{{
django.db.utils.DatabaseError: ORDER BY not allowed in subqueries of compound statements.
}}}

MySQL seems to accept this query but it's most likely not the query the user expects the be executed.

My proposal is to simply return the non-empty query and make the `union` a no-op set in this case."	Bug	closed	Database layer (models, ORM)	3.1	Normal	fixed			Ready for checkin	1	0	0	0	0	0