Create subject counts on collection

db/migrate/20260520193213_add_subjects_count_to_collections.rb

@@ -0,0 +1,8 @@
+class AddSubjectsCountToCollections < ActiveRecord::Migration[7.2]
+  disable_ddl_transaction!
+
+  def change
+    add_column :collections, :subjects_count, :integer, default: 0
+    add_index :collections, :subjects_count, algorithm: :concurrently
+  end
+end

db/structure.sql

@@ -234,7 +234,8 @@ CREATE TABLE public.collections (
     slug character varying DEFAULT ''::character varying,
     favorite boolean DEFAULT false NOT NULL,
     default_subject_id integer,
-    description text DEFAULT ''::text
+    description text DEFAULT ''::text,
+    subjects_count integer DEFAULT 0
 );
 
 
@@ -2823,6 +2824,13 @@ CREATE INDEX index_collections_on_private ON public.collections USING btree (pri
 CREATE INDEX index_collections_on_slug ON public.collections USING btree (slug);
 
 
+--
+-- Name: index_collections_on_subjects_count; Type: INDEX; Schema: public; Owner: -
+--
+
+CREATE INDEX index_collections_on_subjects_count ON public.collections USING btree (subjects_count);
+
+
 --
 -- Name: index_collections_projects_on_collection_id; Type: INDEX; Schema: public; Owner: -
 --
@@ -4240,6 +4248,7 @@ ALTER TABLE ONLY public.users
 SET search_path TO "$user", public;
 
 INSERT INTO "schema_migrations" (version) VALUES
+('20260520193213'),
 ('20260323120200'),
 ('20260323120100'),
 ('20260323120000'),

lib/tasks/collections.rake

@@ -0,0 +1,22 @@
+namespace :collections do
+  desc 'Backfill subjects_count on collections'
+  task backfill_subjects_count: :environment do
+    Collection.in_batches(of: 1_000) do |batch|
+      collection_ids = batch.pluck(:id)
+
+      counts = CollectionSubject
+               .where(collection_id: collection_ids)
+               .group(:collection_id)
+               .count
+
+      collections_to_update = collection_ids.map do |id|
+        [id, counts[id] || 0]
+      end
+
+      collections_to_update.each do |id, count|
+        Collection.where(id: id)
+                  .update_all(subjects_count: count)
+      end
+    end
+  end
+end