Latest event per screen without N+1 using Subquery and Exists

# ======================================================================
# Q: For each screen I want its latest event (e.g. the latest error). How do you
# do it without N+1?
#
# With an index on (screen_id, level, created_at DESC) the database finds the
# latest event per screen without scanning the table. For "the last 3 records" I
# use Prefetch with a window function or, on PostgreSQL, a LATERAL join in raw
# SQL.
# ======================================================================

from django.db.models import OuterRef, Subquery, Exists

last_error = (ScreenEvent.objects
    .filter(screen=OuterRef('pk'), level='error')
    .order_by('-created_at'))

screens = Screen.objects.filter(workspace=ws).annotate(
    last_error_msg=Subquery(last_error.values('message')[:1]),
    last_error_at=Subquery(last_error.values('created_at')[:1]),
    has_errors=Exists(last_error),
)