QuerySet.count() on combined queries with select_related() crashes on MySQL.

[Sunkyue]

test case:

class ModelA(models.Model):
    value = models.IntegerField()

class ModelB(models.Model):
   model_a = models.ForeignKey("ModelA", on_delete=models.CASCADE)

->

ModelB.objects.select_related('model_a').union(ModelB.objects.select_related('model_a')).count()

results in error.

 File "c:\Users\user\Documents\workspace\realclass2-api\.venv\lib\site-packages\django\db\models\query.py", line 412, in count
    return self.query.get_count(using=self.db)
  File "c:\Users\user\Documents\workspace\realclass2-api\.venv\lib\site-packages\django\db\models\sql\query.py", line 521, in get_count
    number = obj.get_aggregation(using, ['__count'])['__count']
  File "c:\Users\user\Documents\workspace\realclass2-api\.venv\lib\site-packages\django\db\models\sql\query.py", line 506, in get_aggregation
    result = compiler.execute_sql(SINGLE)
  File "c:\Users\user\Documents\workspace\realclass2-api\.venv\lib\site-packages\django\db\models\sql\compiler.py", line 1175, in execute_sql
    cursor.execute(sql, params)
  File "c:\Users\user\Documents\workspace\realclass2-api\.venv\lib\site-packages\django\db\backends\utils.py", line 98, in execute
    return super().execute(sql, params)
  File "c:\Users\user\Documents\workspace\realclass2-api\.venv\lib\site-packages\django\db\backends\utils.py", line 66, in execute
    return self._execute_with_wrappers(sql, params, many=False, executor=self._execute)
  File "c:\Users\user\Documents\workspace\realclass2-api\.venv\lib\site-packages\django\db\backends\utils.py", line 75, in _execute_with_wrappers
    return executor(sql, params, many, context)
  File "c:\Users\user\Documents\workspace\realclass2-api\.venv\lib\site-packages\django\db\backends\utils.py", line 84, in _execute
    return self.cursor.execute(sql, params)
  File "c:\Users\user\Documents\workspace\realclass2-api\.venv\lib\site-packages\django\db\utils.py", line 90, in __exit__
    raise dj_exc_value.with_traceback(traceback) from exc_value
  File "c:\Users\user\Documents\workspace\realclass2-api\.venv\lib\site-packages\django\db\backends\utils.py", line 84, in _execute
    return self.cursor.execute(sql, params)
  File "c:\Users\user\Documents\workspace\realclass2-api\.venv\lib\site-packages\django\db\backends\mysql\base.py", line 73, in execute
    return self.cursor.execute(query, args)
  File "c:\Users\user\Documents\workspace\realclass2-api\.venv\lib\site-packages\MySQLdb\cursors.py", line 206, in execute
    res = self._query(query)
  File "c:\Users\user\Documents\workspace\realclass2-api\.venv\lib\site-packages\MySQLdb\cursors.py", line 319, in _query
    db.query(q)
  File "c:\Users\user\Documents\workspace\realclass2-api\.venv\lib\site-packages\MySQLdb\connections.py", line 259, in query
    _mysql.connection.query(self, query)
django.db.utils.OperationalError: (1060, "Duplicate column name 'id'")

reproduced on on 3.2.5/3.2.6

suggested fix:
on django.db.models.sql.query.py -> Query.get_aggregation, from

            inner_query.select_for_update = False
            inner_query.select_related = False
            inner_query.set_annotation_mask(self.annotation_select)

to

            inner_query.select_for_update = False
            inner_query.select_related = False
            for combined_query in inner_query.combined_queries:
                combined_query.select_related = False
            inner_query.set_annotation_mask(self.annotation_select)

can solve the problem I think.

my current monkey-patching code is,

from django.db.models.sql.query import Query

old_get_aggregation = Query.get_aggregation

def get_aggregation(self, using, added_aggregate_names):
    cloned_self = self.clone()
    for combined_query in cloned_self.combined_queries:
        combined_query.select_related = False
    return old_get_aggregation(cloned_self, using, added_aggregate_names)

Query.get_aggregation = get_aggregation

[Mariusz Felisiak]

Thanks for the report. The proposed patch looks good. Would you like to prepare PR?

[Sunkyue]

Replying to Mariusz Felisiak:

Thanks for the report. The proposed patch looks good. Would you like to prepare PR?

Thank you for your review.

I'd like to submit a PR, however I don't have enough time for that for a while. (I haven't submitted any PRs to open source project...so for me, it will took some time..)

If anyone who can submit PR before me, it would be welcomed.

---

In the mean time, I found another buggy case:

ModelB.objects.select_related('model_a').union(ModelB.objects.select_related('model_a')).union(ModelB.objects.select_related('model_a')).count()

produces,

OperationalError: (1222, 'The used SELECT statements have a different number of columns')

because it produces nested configuration like,

query.combined_queries = (query.combined_queries(query, query), query)

So,

            for combined_query in inner_query.combined_queries:
                combined_query.select_related = False

this fix should be improved by doing this in recursive way I think...

[Jordan Bae]

Could i look into and make a patch for this?

[Jordan Bae]

As my investigation, this issue is from subquery's duplicated column. it's not related to aggregate function.
In my opinion, we had better to improve the subquery which includes join. I attached a example for this.

mysql> select * from (
-> select django_migrations.id, dj.id from django_migrations inner join django_migrations as dj on dj.id = django_migrations.id
-> ) subquery;
ERROR 1060 (42S21): Duplicate column name 'id' `

and current SQL is created by Subquery.

a=Subquery(ModelB.objects.select_related('model_a'))
print(a.query)
SELECT blog_modelb.id, blog_modelb.model_a_id, blog_modela.id, blog_modela.value FROM blog_modelb INNER JOIN
blog_modela ON (blog_modelb.model_a_id = blog_modela.id)

If there is any missing or I misunderstood. please know me.
I will try to fix for this.

[Jordan Bae]

Originally, that queryset generates below query.

SELECT
	COUNT(*)
FROM ((
		SELECT
			`blog_modelb`.`id`,
			`blog_modelb`.`model_a_id`,
			`blog_modela`.`id`,
			`blog_modela`.`value`
		FROM
			`blog_modelb`
			INNER JOIN `blog_modela` ON (`blog_modelb`.`model_a_id` = `blog_modela`.`id`))
UNION (
	SELECT
		`blog_modelb`.`id`,
		`blog_modelb`.`model_a_id`,
		`blog_modela`.`id`,
		`blog_modela`.`value`
	FROM
		`blog_modelb`
		INNER JOIN `blog_modela` ON (`blog_modelb`.`model_a_id` = `blog_modela`.`id`))) subquery

@Sunkyue
I think you can use this during shot-term period.

>>> ModelB.objects.select_related('model_a').extra(select={'a': '`blog_modela`.`id`'}).values('id', 'model_a_id', 'a', 'model_a__value').union(ModelB.objects.select_related('model_a').extra(select={'a': '`blog_modela`.`id`'}).values('id', 'model_a_id', 'a', 'model_a__value')).count()
SELECT
	COUNT(*)
FROM ((
		SELECT
			(`blog_modela`.`id`) AS `a`,
			`blog_modelb`.`id`,
			`blog_modelb`.`model_a_id`,
			`blog_modela`.`value`
		FROM
			`blog_modelb`
			INNER JOIN `blog_modela` ON (`blog_modelb`.`model_a_id` = `blog_modela`.`id`))
UNION (
	SELECT
		(`blog_modela`.`id`) AS `a`,
		`blog_modelb`.`id`,
		`blog_modelb`.`model_a_id`,
		`blog_modela`.`value`
	FROM
		`blog_modelb`
		INNER JOIN `blog_modela` ON (`blog_modelb`.`model_a_id` = `blog_modela`.`id`))) subquery

0

your fix looks making an unexpected SQL.

@ Mariusz Felisiak if you have a good idea for this bug, please share.

[Simon Charette]

I haven't tried it out but I wouldn't be surprised if this was fixed by 70499b25c708557fb9ee2264686cd172f4b2354e (#34123) as it ensures that all combined queries have non-colliding aliases.

[Mariusz Felisiak]

Replying to Simon Charette:

I haven't tried it out but I wouldn't be surprised if this was fixed by 70499b25c708557fb9ee2264686cd172f4b2354e (#34123) as it ensures that all combined queries have non-colliding aliases.

Good catch! I submitted ​PR with tests.

Fixed in 70499b25c708557fb9ee2264686cd172f4b2354e.

[GitHub <noreply@…>]

In a411b909:

Refs #33050 -- Added test for QuerySet.count() on combined queries with select_related().

Thanks Simon Charette for noticing this.

Fixed in 70499b25c708557fb9ee2264686cd172f4b2354e.