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
31390	Using Q as rhs value in filter causes SQL Syntax error	Marcin Wieczorek	nobody	"Hello. I discovered this issue by accident. I couldn't find what documentation says about undefined behaviour of filter, so I assume it is a bug.
When a Q object is passed as right hand side value to filter it is treated as allowed, but in the end serializes to empty string. I assume that a one ""word"" AND is simplified to that word, (q AND nothing = q) and also two ""nothings"" ANDed get simplified to empty string. This results in something like ""pk = LIMIT"" in the example.

The model is trivial, it works for anything.
{{{
class A(models.Model):
    pass
}}}

Steps to reproduce
{{{
./manage.py shell
>>> from app.models import *
>>> from django.db.models import Q
>>> A.objects.filter(pk=Q())
Traceback (most recent call last):
  File ""/secret/venv37/lib/python3.7/site-packages/django/db/backends/utils.py"", line 86, in _execute
    return self.cursor.execute(sql, params)
  File ""/secret/venv37/lib/python3.7/site-packages/django/db/backends/sqlite3/base.py"", line 396, in execute
    return Database.Cursor.execute(self, query, params)
sqlite3.OperationalError: near ""LIMIT"": syntax error

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File ""<console>"", line 1, in <module>
  File ""/secret/venv37/lib/python3.7/site-packages/django/db/models/query.py"", line 252, in __repr__
    data = list(self[:REPR_OUTPUT_SIZE + 1])
  File ""/secret/venv37/lib/python3.7/site-packages/django/db/models/query.py"", line 276, in __iter__
    self._fetch_all()
  File ""/secret/venv37/lib/python3.7/site-packages/django/db/models/query.py"", line 1261, in _fetch_all
    self._result_cache = list(self._iterable_class(self))
  File ""/secret/venv37/lib/python3.7/site-packages/django/db/models/query.py"", line 57, in __iter__
    results = compiler.execute_sql(chunked_fetch=self.chunked_fetch, chunk_size=self.chunk_size)
  File ""/secret/venv37/lib/python3.7/site-packages/django/db/models/sql/compiler.py"", line 1151, in execute_sql
    cursor.execute(sql, params)
  File ""/secret/venv37/lib/python3.7/site-packages/django/db/backends/utils.py"", line 100, in execute
    return super().execute(sql, params)
  File ""/secret/venv37/lib/python3.7/site-packages/django/db/backends/utils.py"", line 68, in execute
    return self._execute_with_wrappers(sql, params, many=False, executor=self._execute)
  File ""/secret/venv37/lib/python3.7/site-packages/django/db/backends/utils.py"", line 77, in _execute_with_wrappers
    return executor(sql, params, many, context)
  File ""/secret/venv37/lib/python3.7/site-packages/django/db/backends/utils.py"", line 86, in _execute
    return self.cursor.execute(sql, params)
  File ""/secret/venv37/lib/python3.7/site-packages/django/db/utils.py"", line 90, in __exit__
    raise dj_exc_value.with_traceback(traceback) from exc_value
  File ""/secret/venv37/lib/python3.7/site-packages/django/db/backends/utils.py"", line 86, in _execute
    return self.cursor.execute(sql, params)
  File ""/secret/venv37/lib/python3.7/site-packages/django/db/backends/sqlite3/base.py"", line 396, in execute
    return Database.Cursor.execute(self, query, params)
django.db.utils.OperationalError: near ""LIMIT"": syntax error
}}}

Bonus:
{{{
>>> A.objects.filter(pk=Q()).query.__str__()
'SELECT ""app_a"".""id"" FROM ""app_a"" WHERE ""app_a"".""id"" = '
}}}


Notes:
If this gets confirmed I'm willing to try to do a fix (plenty of time nowadays, right?), or at least provide a failing test, because as far as I dug into the code, the query interfaces are fairly complicated.

This example was made on sqlite, but the issue was originally discovered on postgres. I assume the bug is not on db backend level.

As absurd as this code might look, I created it by adding an inline if statemement to the filter (pk=1 if flag else Q()), which wasn't an obvious error."	Bug	closed	Database layer (models, ORM)	3.0	Normal	invalid	Q filter SQL		Unreviewed	0	0	0	0	0	0