Syntax Error when combining __in and F() in filter

[Douglas Franklin]

Similar to https://code.djangoproject.com/ticket/32151

I have a simple Google Calendar Events application. Profiles generate calendar tokens with GSuite Oauth (not shown) that are used to make CalendarEvents in my database (postgres:9.6). CalendarEvents queried via a particular CalendarToken have the token's profile on the model. See oversimplified example below:

class Profile(models.Model):
    name = models.CharField(max_length=32)

class CalendarEvent(models.Model):
   profile = models.ForeignKey(Profile, on_delete=models.CASCADE)
   attendees = models.ManyToManyField(Profile, blank=True, related_name="attending_calendar_events")

I'm trying to annotate whether a profile is "attending" the event, by checking if the profile is in the attendees ManyToMany:

CalendarEvent.objects.annotate(
    is_attending=Case(
        When(
            profile__in=F('attendees'),
            then=Value(True)
        ),
        default=Value(False),
        output_field=models.BooleanField()
    )
)

Generates an invalid syntax error (replacing db name with {APP}):

ProgrammingError: syntax error at or near ""{APP}_calendarevent_attendees""
LINE 1: ...CASE WHEN ("{APP}_calendarevent"."profile_id" IN "{APP}_ca...

[Simon Charette]

Looks like you either want attendees=F('profile') or profile__in=CalendarEvent.attendees.through.objects.filter(calendarevent=OuterRef('pk')).

I guess we could raise a Python level error or adjust RelatedIn.get_prep_lookup special case F referring to many-to-many as well.

[Douglas Franklin]

The latter worked perfectly. Really interesting. Thank you! I agree though, seems like a practical use-case to have the special case when get_prep_lookup F refers to a many-to-many.

The use of the through-table subquery is working for me, so it seems like an easy enough workaround.

Replying to Simon Charette:

Looks like you either want attendees=F('profile') or profile__in=CalendarEvent.attendees.through.objects.filter(calendarevent=OuterRef('pk')).values('profile').

I guess we could raise a Python level error or adjust RelatedIn.get_prep_lookup special case F referring to many-to-many as well.

[Mariusz Felisiak]

Duplicate of #31135. Please feel-free to add a comment to the original ticket.