manage.py error after Wagtail 2.15 upgrade

After upgrading to Wagtail 2.15 (or 2.15.1) from 2.14.2 my production website with postgres and database search breaks and commands that are run with manage.py give an error despite me adding the required WAGTAILSEARCH_BACKENDS to settings.

I have two web apps with separate settings running from the same Wagtail version. One of the apps (putkeep) has a search bar and the other (secretgifter) does not. After upgrading Wagtail from 2.14.2 to 2.15, putkeep gives a 404 error but secretgifter does not. If I use pip to switch back to 2.14.2, then the 404 error goes away and the site loads (although results from a search give a 500 error).

If I run makemigrations (or any other command that uses manage.py) for secretgifter it works fine. For putkeep (with the search) it gives the following error:

  File "manage.py", line 10, in <module>
    execute_from_command_line(sys.argv)
  File "/home/th-putkeep.net/putkeep/lib/python3.8/site-packages/django/core/management/__init__.py", line 419, in execute_from_command_line
    utility.execute()
  File "/home/th-putkeep.net/putkeep/lib/python3.8/site-packages/django/core/management/__init__.py", line 395, in execute
    django.setup()
  File "/home/th-putkeep.net/putkeep/lib/python3.8/site-packages/django/__init__.py", line 24, in setup
    apps.populate(settings.INSTALLED_APPS)
  File "/home/th-putkeep.net/putkeep/lib/python3.8/site-packages/django/apps/registry.py", line 122, in populate
    app_config.ready()
  File "/home/th-putkeep.net/putkeep/lib/python3.8/site-packages/wagtail/search/apps.py", line 21, in ready
    set_weights()
  File "/home/th-putkeep.net/putkeep/lib/python3.8/site-packages/wagtail/search/backends/database/postgres/weights.py", line 44, in set_weights
    BOOSTS_WEIGHTS.extend(determine_boosts_weights())
  File "/home/th-putkeep.net/putkeep/lib/python3.8/site-packages/wagtail/search/backends/database/postgres/weights.py", line 32, in determine_boosts_weights
    boosts = get_boosts()
  File "/home/th-putkeep.net/putkeep/lib/python3.8/site-packages/wagtail/search/backends/database/postgres/weights.py", line 26, in get_boosts
    boosts.add(boost)
TypeError: unhashable type: 'list'

As per the docs I've added this to my settings:

WAGTAILSEARCH_BACKENDS = {
    'default': {
        'BACKEND': 'wagtail.search.backends.database',
    }
}

Any suggestions gratefully received.

Solution

I have identified some code in my models.py that caused no errors with my site running Wagtail 2.14.2 and below. When commented out, it resolves the error caused by upgrading to Wagtail 2.15 and above. I am posting it here as the answer to my problem because everything else seems to work (including search) without any further modification even though I am not currently sure why it causes the error or if I need it anymore:

    search_fields = Page.search_fields + [  # Inherit search_fields from Page
        index.SearchField('content'),
        index.SearchField('tags', [
            index.SearchField('name', partial_match=True, boost=10),
        ]),
    ]