Elasticsearch backend stacktrace with order_by_relevance=False

[nutjob4life]

Issue Summary

When a query set has certain order_by conditions added to it, such as Lower('some_field'), the Elasticsearch backend produces a stack trace when order_by_relevance=False is used to search the set.

Steps to Reproduce

$ date -u
Fri Jul 15 13:58:47 UTC 2022
$ sw_vers
ProductName:    macOS
ProductVersion: 12.4
BuildVersion:   21F79
$ python3 --version
Python 3.10.5
$ python3 -m venv venv
$ . venv/bin/activate
(venv) $ pip install --quiet --upgrade wagtail==3.0.1 pip setuptools
(venv) $ wagtail start issue
Creating a Wagtail project called issue
Success! issue has been created
(venv) $ cd issue
(venv) $ echo 'elasticsearch~=7.17' >> requirements.txt
(venv) $ pip install --quiet --requirement requirements.txt
(venv) $ cat >>issue/settings/base.py <<EOF
WAGTAILSEARCH_BACKENDS = {
    'default': {
        'ATOMIC_REBUILD': True,
        'AUTO_UPDATE': True,
        'BACKEND': 'wagtail.search.backends.elasticsearch7',
        'INDEX': 'wagtail',
        'INDEX_SETTINGS': {},
        'OPTIONS': {},
        'TIMEOUT': 5,
        'URLS': ['http://localhost:9200'],
    }
}
EOF
(venv) $ ./manage.py migrate
Operations to perform:
  Apply all migrations: admin, auth, contenttypes, home, sessions, taggit, wagtailadmin, wagtailcore, wagtaildocs, wagtailembeds, wagtailforms, wagtailimages, wagtailredirects, wagtailsearch, wagtailusers
…
  Applying wagtailusers.0009_userprofile_verbose_name_plural... OK
  Applying wagtailusers.0010_userprofile_updated_comments_notifications... OK
(venv) $ ./manage.py createsuperuser --no-input --username admin --email admin@admin.com
Superuser created successfully.
(venv) $ ./manage.py shell <<EOF
from wagtail.core.models import Page, Site
from django.db.models.functions import Lower
root = Site.objects.filter(is_default_site=True).first().root_page
p1 = Page(title='hello world')
root.add_child(instance=p1)
p1.save()
p2 = Page(title='Hello World')
root.add_child(instance=p2)
p2.save()
Page.objects.all().order_by(Lower('title')).search('world', order_by_relevance=False)
EOF
Traceback (most recent call last):
  File "issue/./manage.py", line 10, in <module>
    execute_from_command_line(sys.argv)
…
  File "venv/lib/python3.10/site-packages/wagtail/search/backends/base.py", line 174, in _get_order_by
    if field_name.startswith("-"):
AttributeError: 'Lower' object has no attribute 'startswith'

Any other relevant information. For example, why do you consider this a bug and what did you expect to happen instead?

It feels like a bug because the documentation at this page states that order_by_relevance=False preserves the QuerySet's ordering:

• I have confirmed that this issue can be reproduced as described on a fresh Wagtail project: yes

Technical details

• Python version: 3.10.5
• Django version: 4.0.6
• Wagtail version: 3.0.1
• Browser version: N/A

[NXPY123]

I was able to reproduce the issue by following the steps mentioned by @nutjob4life

Steps to Reproduce

source ./WagTail/WagTail/bin/activate
$ date -u
Fri Sep 29 04:01:28 UTC 2023
$ sw_vers          
ProductName:            macOS
ProductVersion:         13.3.1
ProductVersionExtra:    (a)
BuildVersion:           22E772610a
$ python3 --version
Python 3.11.4
$ pip install --quiet --upgrade wagtail                      
$ pip install --upgrade setuptools
$ wagtail start issue
$ cd issue
$ echo 'elasticsearch~=7.17' >> requirements.txt
$ pip install --quiet --requirement requirements.txt
$ cat >>issue/settings/base.py <<EOF
heredoc> WAGTAILSEARCH_BACKENDS = {
    'default': {
        'ATOMIC_REBUILD': True,
        'AUTO_UPDATE': True,
        'BACKEND': 'wagtail.search.backends.elasticsearch7',
        'INDEX': 'wagtail',
        'INDEX_SETTINGS': {},
        'OPTIONS': {},
        'TIMEOUT': 5,
        'URLS': ['http://localhost:9200'],
    }
}
EOF
$ ./manage.py migrate
$ ./manage.py createsuperuser --no-input --username admin --email admin@admin.com
Superuser created successfully.
$ ./manage.py shell <<EOF
from wagtail.models import Page, Site 
from django.db.models.functions import Lower
root = Site.objects.filter(is_default_site=True).first().root_page
p1 = Page(title='hello world')
root.add_child(instance=p1)
p1.save()
p2 = Page(title='Hello World')
root.add_child(instance=p2)
p2.save()
Page.objects.all().order_by(Lower('title')).search('world', order_by_relevance=False)
EOF
Exception raised while adding <Page: hello world> into the 'default' search backend
Traceback (most recent call last):
  File "/Users/neeraj_py/Desktop/WagTail/issue/./manage.py", line 10, in <module>
    execute_from_command_line(sys.argv)
...
  File "/Users/neeraj_py/Desktop/WagTail/WagTail/lib/python3.11/site-packages/wagtail/search/backends/base.py", line 233, in _get_order_by
    if field_name.startswith("-"):
       ^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'Lower' object has no attribute 'startswith'

Technical Details

• Python: 3.11.4
• Django: 4.2.5
• Wagtail: 5.1.2