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
15854	"Combining two querysets generated by ""extra"" causes duplication in extra_tables"	cwu	nobody	"I am using Django v1.2.3 with Oracle DB backend. The code below demo the possible bug I found:

{{{
#I have two models
#model_1
#model_2
tbl1 = model_1._meta.db_table
tbl2 = model_2._meta.db_table
#let's say
#tbl1='tbl1'
#tbl2='tbl2'
qs1 = model_1.objects.extra(tables=[tbl2], where=[""%s.field1=%s.field2"" % (tbl1, tbl2)])        
qs2 = model_1.objects.extra(tables=[tbl2], where=[""%s.field3=%s.field4"" % (tbl1, tbl2)])
qs3 = qs1 | qs2
}}}

Both qs1 and qs2 are built from ""extra"" by including the same tbl2. Then when I took the union of two (qs3), I got Oracle error ""ORA-00918: column ambiguously defined"". I dug into the django db code, and realized the problem was because qs3.query.extra_tables contains duplicated ""tbl2"" after combining (taking union) ""qs1"" and ""qs2"", which further causes duplicated tbl2 in from clause:

{{{
In [44]: qs3.query.extra_tables
Out[44]: ('tbl2', 'tbl2')

In [43]: qs3._as_sql(connection)
Out[43]:
('SELECT TBL1.""ID"" FROM ""TBL1"" , ""TBL2"" , ""TBL2"" WHERE (tbl1.field1=tbl2.field2 OR tbl1.field3=tbl2.field4)',
 ())

}}}

I am not sure if this will cause problem in other DB types, but clearly Oracle does not like it. I ended up fixing it by removing duplication after taking union, but before the query was executed.

{{{
qs3.query.extra_tables = tuple(set(qs3.query.extra_tables))
}}}

To actual fix this issue from Django side, I guess uniquness-checking can be either in 
{{{
django.db.models.sql.query.Query.combine
}}}
or
{{{
django.db.models.sql.compiler.SQLCompiler.get_from_clause
}}}

or may be both, just to make sure the generated ""from"" clause doesn't contain duplicated tables.

I double-checked that I can reproduce this problem in either v1.2.5 or v1.3 ."	Bug	closed	Database layer (models, ORM)	1.2	Normal	wontfix	db sql union extra		Unreviewed	0	0	0	0	0	0