Wipe old metadata from link topics on link change

This is a bit ugly (and probably the wrong spot to do it), but we need
to wipe the old metadata before adding in new stuff from the
re-scrape(s). Otherwise, if we do something like change a YouTube video
to an article, the duration of the video will still be left in the
metadata even though it's no longer relevant.

tildes/tildes/lib/link_metadata.py

@@ -0,0 +1,13 @@
+# Copyright (c) 2019 Tildes contributors <code@tildes.net>
+# SPDX-License-Identifier: AGPL-3.0-or-later
+
+"""Constants/classes/functions related to metadata generated from links."""
+
+METADATA_KEYS = [
+    "authors",
+    "description",
+    "duration",
+    "published",
+    "title",
+    "word_count",
+]

tildes/tildes/views/api/web/topic.py

@@ -8,10 +8,13 @@ from marshmallow.fields import String
 from pyramid.httpexceptions import HTTPNotFound
 from pyramid.response import Response
 from pyramid.request import Request
+from sqlalchemy import cast, Text
+from sqlalchemy.dialects.postgresql import ARRAY
 from sqlalchemy.exc import IntegrityError
 from webargs.pyramidparser import use_kwargs
 
 from tildes.enums import LogEventType
+from tildes.lib.link_metadata import METADATA_KEYS
 from tildes.models.group import Group
 from tildes.models.log import LogTopic
 from tildes.models.topic import Topic, TopicBookmark, TopicVote
@@ -350,6 +353,21 @@ def patch_topic_link(request: Request, link: str) -> dict:
         )
     )
 
+    # Wipe any old metadata from scrapers so we don't leave behind remnants
+    # (this probably really shouldn't be done here, but it's fine for now)
+    (
+        request.query(Topic)
+        .filter(Topic.topic_id == topic.topic_id)
+        .update(
+            {
+                "content_metadata": Topic.content_metadata.op("-")(  # type: ignore
+                    cast(METADATA_KEYS, ARRAY(Text))
+                )
+            },
+            synchronize_session=False,
+        )
+    )
+
     topic.link = link
 
     return Response(f'<a href="{topic.link}">{topic.link}</a>')