Topic metadata generator: update, don't replace

Previously, any topic processed by this consumer would have its
content_metadata completely replaced. This won't work once other
consumers or processes start being able to set that data, since we don't
know that this one will always run first.

This commit updates the method the consumer uses so that it will keep
any data that's already in the topic's content_metadata column if
necessary. It would probably be good to generalize this method out
somehow so that it can be used in other places more easily.

tildes/consumers/topic_metadata_generator.py

@@ -3,10 +3,12 @@
 
 """Consumer that generates content_metadata for topics."""
 
-from typing import Sequence
+from typing import Any, Dict, Sequence
 
 from amqpy import Message
 import publicsuffix
+from sqlalchemy import cast, func
+from sqlalchemy.dialects.postgresql import JSONB
 
 from tildes.lib.amqp import PgsqlQueueConsumer
 from tildes.lib.string import extract_text_from_html, truncate_string, word_count
@@ -32,32 +34,41 @@ class TopicMetadataGenerator(PgsqlQueueConsumer):
         )
 
         if topic.is_text_type:
-            self._generate_text_metadata(topic)
+            new_metadata = self._generate_text_metadata(topic)
         elif topic.is_link_type:
-            self._generate_link_metadata(topic)
+            new_metadata = self._generate_link_metadata(topic)
+
+        # update the topic's content_metadata in a way that won't wipe out any existing
+        # values, and can handle the column being null
+        (
+            self.db_session.query(Topic)
+            .filter(Topic.topic_id == topic.topic_id)
+            .update(
+                {
+                    "content_metadata": func.coalesce(
+                        Topic.content_metadata, cast({}, JSONB)
+                    ).op("||")(new_metadata)
+                },
+                synchronize_session=False,
+            )
+        )
 
     @staticmethod
-    def _generate_text_metadata(topic: Topic) -> None:
+    def _generate_text_metadata(topic: Topic) -> Dict[str, Any]:
         """Generate metadata for a text topic (word count and excerpt)."""
         extracted_text = extract_text_from_html(topic.rendered_html)
 
         # create a short excerpt by truncating the extracted string
         excerpt = truncate_string(extracted_text, length=200, truncate_at_chars=" ")
 
-        topic.content_metadata = {
-            "word_count": word_count(extracted_text),
-            "excerpt": excerpt,
-        }
+        return {"word_count": word_count(extracted_text), "excerpt": excerpt}
 
-    def _generate_link_metadata(self, topic: Topic) -> None:
+    def _generate_link_metadata(self, topic: Topic) -> Dict[str, Any]:
         """Generate metadata for a link topic (domain)."""
-        if not topic.link:
-            return
-
         parsed_domain = get_domain_from_url(topic.link)
         domain = self.public_suffix_list.get_public_suffix(parsed_domain)
 
-        topic.content_metadata = {"domain": domain}
+        return {"domain": domain}
 
 
 if __name__ == "__main__":