DEV: Add polymorphic bookmarkable columns (#15454)

We are planning on attaching bookmarks to more and
more other models, so it makes sense to make a polymorphic
relationship to handle this. This commit adds the new
columns and backfills them in the bookmark table, and
makes sure that any new bookmark changes fill in the columns
via DB triggers.

This way we can gradually change the frontend and backend
to use these new columns, and eventually delete the
old post_id and for_topic columns in `bookmarks`.

db/migrate/20220104053343_add_bookmark_polymorphic_columns.rb

@@ -0,0 +1,10 @@
+# frozen_string_literal: true
+
+class AddBookmarkPolymorphicColumns < ActiveRecord::Migration[6.1]
+  def change
+    add_column :bookmarks, :bookmarkable_id, :integer
+    add_column :bookmarks, :bookmarkable_type, :string
+
+    add_index :bookmarks, [:user_id, :bookmarkable_type, :bookmarkable_id], name: "idx_bookmarks_user_polymorphic_unique", unique: true
+  end
+end

db/migrate/20220105024605_add_trigger_for_polymorphic_bookmark_columns_to_sync_data.rb

@@ -0,0 +1,48 @@
+# frozen_string_literal: true
+
+class AddTriggerForPolymorphicBookmarkColumnsToSyncData < ActiveRecord::Migration[6.1]
+  def up
+    DB.exec <<~SQL
+      CREATE OR REPLACE FUNCTION sync_bookmarks_polymorphic_column_data()
+      RETURNS TRIGGER
+      LANGUAGE PLPGSQL AS $rcr$
+      BEGIN
+        IF NEW.for_topic
+        THEN
+          NEW.bookmarkable_id = (SELECT topic_id FROM posts WHERE posts.id = NEW.post_id);
+          NEW.bookmarkable_type = 'Topic';
+        ELSE
+          NEW.bookmarkable_id = NEW.post_id;
+          NEW.bookmarkable_type = 'Post';
+        END IF;
+        RETURN NEW;
+      END
+      $rcr$;
+    SQL
+
+    DB.exec <<~SQL
+      CREATE TRIGGER bookmarks_polymorphic_data_sync
+      BEFORE INSERT OR UPDATE OF post_id, for_topic ON bookmarks
+      FOR EACH ROW
+      EXECUTE FUNCTION sync_bookmarks_polymorphic_column_data();
+    SQL
+
+    # sync data that already exists in the table
+    DB.exec(<<~SQL)
+      UPDATE bookmarks
+      SET bookmarkable_id = post_id, bookmarkable_type = 'Post'
+      WHERE NOT bookmarks.for_topic
+    SQL
+    DB.exec(<<~SQL)
+      UPDATE bookmarks
+      SET bookmarkable_id = posts.topic_id, bookmarkable_type = 'Topic'
+      FROM posts
+      WHERE bookmarks.for_topic AND posts.id = bookmarks.post_id
+    SQL
+  end
+
+  def down
+    DB.exec("DROP TRIGGER IF EXISTS bookmarks_polymorphic_data_sync")
+    DB.exec("DROP FUNCTION IF EXISTS sync_bookmarks_polymorphic_column_data")
+  end
+end