FIX: Backfill chat thread memberships (#21971)

Since we created user_chat_thread_memberships in
cc2570f we haven't
yet backfilled it for users who previously sent a message in
in threads -- this migration creates the UserChatThreadMemberships
needed for those threads, making sure the last read message id
is accurate for those participants.

plugins/chat/app/models/chat/user_chat_thread_membership.rb

@@ -20,7 +20,11 @@ end
 #  user_id              :bigint           not null
 #  thread_id            :bigint           not null
 #  last_read_message_id :bigint
-#  notification_level   :integer          default(2), not null
+#  notification_level   :integer          default("tracking"), not null
 #  created_at           :datetime         not null
 #  updated_at           :datetime         not null
 #
+# Indexes
+#
+#  user_chat_thread_unique_memberships  (user_id,thread_id) UNIQUE
+#

plugins/chat/db/migrate/20230607091233_backfill_thread_memberships.rb

@@ -0,0 +1,51 @@
+# frozen_string_literal: true
+
+class BackfillThreadMemberships < ActiveRecord::Migration[7.0]
+  def up
+    thread_tracking_notification_level = 2
+
+    sql = <<~SQL
+      INSERT INTO user_chat_thread_memberships(
+        user_id,
+        thread_id,
+        notification_level,
+        last_read_message_id,
+        created_at,
+        updated_at
+      )
+      SELECT
+        thread_participant_stats.user_id,
+        thread_participant_stats.thread_id,
+        #{thread_tracking_notification_level},
+        (
+          SELECT id FROM chat_messages
+          WHERE thread_id = thread_participant_stats.thread_id
+          AND deleted_at IS NULL
+          ORDER BY created_at DESC, id DESC
+          LIMIT 1
+        ),
+        NOW(),
+        NOW()
+      FROM (
+        SELECT chat_messages.thread_id, chat_messages.user_id
+        FROM chat_messages
+        INNER JOIN chat_threads ON chat_threads.id = chat_messages.thread_id
+        WHERE chat_messages.thread_id IS NOT NULL
+        GROUP BY chat_messages.thread_id, chat_messages.user_id
+        ORDER BY chat_messages.thread_id ASC, chat_messages.user_id ASC
+      ) AS thread_participant_stats
+      INNER JOIN users ON users.id = thread_participant_stats.user_id
+      LEFT JOIN user_chat_thread_memberships ON user_chat_thread_memberships.thread_id = thread_participant_stats.thread_id
+        AND user_chat_thread_memberships.user_id = thread_participant_stats.user_id
+      WHERE user_chat_thread_memberships IS NULL
+      ORDER BY user_chat_thread_memberships.thread_id ASC
+      ON CONFLICT DO NOTHING;
+    SQL
+
+    execute(sql)
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end