Using Q as rhs value in filter causes SQL Syntax error

[Marcin Wieczorek]

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.

[Carlton Gibson]

Using Q as rhs value in filter...

Sorry, this is just a misuse of Q. A Q represents the expression, not just the rhs (or lhs).

Look at ​the examples from the docs: Q(question__startswith='What').

So in your case you need A.objects.filter(Q()) (not pk=Q())

Please use support channels if you have further queries.

[Marcin Wieczorek]

I know it's a misuse, I don't expect such code to work. I just thought an exception should be raised, an SQL error could potentially do more damage. As I said, I caused the error in a situation that IDE/parser/runtime couldn't detect the issue until the SQL error (so the misuse wasn't obvious). An exception is raised when you pass a string instead of an int, so should be for Qs.

Anyway thanks for your input, I won't seek further justification for my issue if you say it's invalid.