Translated preview crashes when ChooserBlock's related object does not exist anymore

[th3hamm0r]

Normally, if you use a ChooserBlock (like a page chooser) in a StreamField, if the chosen page gets deleted, the field still renders in the frontend and does not crash.
In the backend, if it is a required field, the now empty chooser would trigger a validation error.

Unfortunately, this is not the case with wagtail-localize and translated pages/objects. If for example the chosen page gets deleted, the preview crashes with the following error:

2024-12-20 08:23:23 ERROR django.request: Internal Server Error: /admin/content/localize/translate/2/preview/ (log.py:241)
Traceback (most recent call last):
  File "/***/lib/python3.11/site-packages/django/core/handlers/exception.py", line 55, in inner
    response = get_response(request)
               ^^^^^^^^^^^^^^^^^^^^^
  File "/***/lib/python3.11/site-packages/django/core/handlers/base.py", line 197, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/***/lib/python3.11/site-packages/sentry_sdk/integrations/django/views.py", line 84, in sentry_wrapped_callback
    return callback(request, *args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/***/lib/python3.11/site-packages/django/views/decorators/cache.py", line 62, in _wrapper_view_func
    response = view_func(request, *args, **kwargs)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/***/lib/python3.11/site-packages/wagtail/admin/urls/__init__.py", line 173, in wrapper
    return view_func(request, *args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/***/lib/python3.11/site-packages/wagtail/admin/auth.py", line 165, in decorated_view
    response = view_func(request, *args, **kwargs)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/***/lib/python3.11/site-packages/django/utils/decorators.py", line 46, in _wrapper
    return bound_method(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/***/lib/python3.11/site-packages/wagtail/utils/decorators.py", line 78, in wrapped_view
    resp = view_func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/***/lib/python3.11/site-packages/wagtail_localize/views/edit_translation.py", line 1134, in preview_translation
    translation = translation.source.get_ephemeral_translated_instance(
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/***/lib/python3.11/site-packages/wagtail_localize/models.py", line 897, in get_ephemeral_translated_instance
    ingest_segments(original, translation, self.locale, locale, segments)
  File "/***/lib/python3.11/site-packages/wagtail_localize/segments/ingest.py", line 263, in ingest_segments
    ).handle_stream_block(data, field_segments)
      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/***/lib/python3.11/site-packages/wagtail_localize/segments/ingest.py", line 228, in handle_stream_block
    block.value = self.handle_block(block.block, block.value, segments)
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/***/lib/python3.11/site-packages/wagtail_localize/segments/ingest.py", line 161, in handle_block
    return self.handle_related_object_block(block_value, segments)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/***/lib/python3.11/site-packages/wagtail_localize/segments/ingest.py", line 178, in handle_related_object_block
    return handle_related_object(
           ^^^^^^^^^^^^^^^^^^^^^^
  File "/***/lib/python3.11/site-packages/wagtail_localize/segments/ingest.py", line 90, in handle_related_object
    return related_model.objects.get(pk=segment.data)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/***/lib/python3.11/site-packages/django/db/models/manager.py", line 87, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/***/lib/python3.11/site-packages/django/db/models/query.py", line 637, in get
    raise self.model.DoesNotExist(
wagtail.models.Page.DoesNotExist: Page matching query does not exist.
[20/Dec/2024 08:23:23] "GET /admin/content/localize/translate/2/preview/ HTTP/1.1" 500 150427

I would expect that the preview still works, but the value is handled as None similar to how Wagtail's ChooserBlock handles it here.

In the UI the translated chooser block stays in the "Fetching page information..." state, because the API request fails with a 404.
Clicking the publish button also leads to a related crash:

2024-12-20 08:30:31 ERROR django.request: Internal Server Error: /admin/content/pages/18/edit/ (log.py:241)
Traceback (most recent call last):
  File "/***/lib/python3.11/site-packages/django/core/handlers/exception.py", line 55, in inner
    response = get_response(request)
               ^^^^^^^^^^^^^^^^^^^^^
  File "/***/lib/python3.11/site-packages/django/core/handlers/base.py", line 197, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/***/lib/python3.11/site-packages/sentry_sdk/integrations/django/views.py", line 84, in sentry_wrapped_callback
    return callback(request, *args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/***/lib/python3.11/site-packages/django/views/decorators/cache.py", line 62, in _wrapper_view_func
    response = view_func(request, *args, **kwargs)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/***/lib/python3.11/site-packages/wagtail/admin/urls/__init__.py", line 173, in wrapper
    return view_func(request, *args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/***/lib/python3.11/site-packages/wagtail/admin/auth.py", line 165, in decorated_view
    response = view_func(request, *args, **kwargs)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/***/lib/python3.11/site-packages/django/views/generic/base.py", line 104, in view
    return self.dispatch(request, *args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/***/lib/python3.11/site-packages/wagtail/admin/views/pages/edit.py", line 375, in dispatch
    response = self.run_hook("before_edit_page", self.request, self.page)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/***/lib/python3.11/site-packages/wagtail/admin/views/generic/mixins.py", line 49, in run_hook
    result = fn(*args, **kwargs)
             ^^^^^^^^^^^^^^^^^^^
  File "/***/lib/python3.11/site-packages/wagtail_localize/wagtail_hooks.py", line 268, in before_edit_page
    return edit_translation.edit_translation(request, translation, page)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/***/lib/python3.11/site-packages/wagtail_localize/views/edit_translation.py", line 565, in edit_translation
    translation.save_target(user=request.user, publish=True)
  File "/***/lib/python3.11/site-packages/wagtail_localize/models.py", line 1344, in save_target
    self.source.create_or_update_translation(
  File "/***/lib/python3.11/site-packages/wagtail_localize/models.py", line 760, in create_or_update_translation
    ingest_segments(original, translation, self.locale, locale, segments)
  File "/***/lib/python3.11/site-packages/wagtail_localize/segments/ingest.py", line 263, in ingest_segments
    ).handle_stream_block(data, field_segments)
      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/***/lib/python3.11/site-packages/wagtail_localize/segments/ingest.py", line 228, in handle_stream_block
    block.value = self.handle_block(block.block, block.value, segments)
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/***/lib/python3.11/site-packages/wagtail_localize/segments/ingest.py", line 161, in handle_block
    return self.handle_related_object_block(block_value, segments)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/***/lib/python3.11/site-packages/wagtail_localize/segments/ingest.py", line 178, in handle_related_object_block
    return handle_related_object(
           ^^^^^^^^^^^^^^^^^^^^^^
  File "/***/lib/python3.11/site-packages/wagtail_localize/segments/ingest.py", line 90, in handle_related_object
    return related_model.objects.get(pk=segment.data)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/***/lib/python3.11/site-packages/django/db/models/manager.py", line 87, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/***/lib/python3.11/site-packages/django/db/models/query.py", line 637, in get
    raise self.model.DoesNotExist(
wagtail.models.Page.DoesNotExist: Page matching query does not exist.
[20/Dec/2024 08:30:31] "POST /admin/content/pages/18/edit/ HTTP/1.1" 500 167964

I would expect a validation error or at least no crash and an empty chooser.

[zerolab]

Thanks @th3hamm0r
This is similar to #769

[th3hamm0r]

Sry, missed that during my search 😅
I've now tested it with the ImageChooserBlock too and it produces the same issue as above. Maybe the error has changed from breaking the UI to the 500 during preview/publish(?)

At least for the backend part, replacing

wagtail-localize/wagtail_localize/segments/ingest.py

Lines 184 to 187 in 7445433

	def handle_related_object_block(self, related_object, segments):
	return handle_related_object(
	related_object.__class__, self.src_locale, self.tgt_locale, segments
	)

with

    def handle_related_object_block(self, related_object, segments):
        try:
            return handle_related_object(
                related_object.__class__, self.src_locale, self.tgt_locale, segments
            )
        except related_object.__class__.DoesNotExist:
            return None

avoids the 500 errors. Of course, this is just a partial fix, but at least the preview and publishing works (in our case) 😬

edit: I have to admit, this is just a quick fix, I'm not sure if the other case where handle_related_object() is used (here) would need that fix too. In that case the try-except would be better placed in handle_related_object()...