Add counter cache to collections (#37176)

app/controllers/api/v1_alpha/collections_controller.rb

@@ -71,7 +71,6 @@ class Api::V1Alpha::CollectionsController < Api::BaseController
   def set_collections
     @collections = @account.collections
                            .with_tag
-                           .with_item_count
                            .order(created_at: :desc)
                            .offset(offset_param)
                            .limit(limit_param(DEFAULT_COLLECTIONS_LIMIT))

app/models/collection.rb

@@ -7,6 +7,7 @@
 #  id                       :bigint(8)        not null, primary key
 #  description              :text             not null
 #  discoverable             :boolean          not null
+#  item_count               :integer          default(0), not null
 #  local                    :boolean          not null
 #  name                     :string           not null
 #  original_number_of_items :integer
@@ -39,11 +40,6 @@ class Collection < ApplicationRecord
   validate :items_do_not_exceed_limit
 
   scope :with_items, -> { includes(:collection_items).merge(CollectionItem.with_accounts) }
-  scope :with_item_count, lambda {
-    select('collections.*, COUNT(collection_items.id)')
-      .left_joins(:collection_items)
-      .group(collections: :id)
-  }
   scope :with_tag, -> { includes(:tag) }
 
   def remote?

app/models/collection_item.rb

@@ -17,7 +17,7 @@
 #  collection_id             :bigint(8)        not null
 #
 class CollectionItem < ApplicationRecord
-  belongs_to :collection
+  belongs_to :collection, counter_cache: :item_count
   belongs_to :account, optional: true
 
   enum :state,

app/serializers/rest/base_collection_serializer.rb

@@ -9,8 +9,4 @@ class REST::BaseCollectionSerializer < ActiveModel::Serializer
   def id
     object.id.to_s
   end
-
-  def item_count
-    object.respond_to?(:item_count) ? object.item_count : object.collection_items.count
-  end
 end

db/migrate/20251209093813_add_item_count_to_collections.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class AddItemCountToCollections < ActiveRecord::Migration[8.0]
+  def change
+    add_column :collections, :item_count, :integer, default: 0, null: false
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[8.0].define(version: 2025_12_02_140424) do
+ActiveRecord::Schema[8.0].define(version: 2025_12_09_093813) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_catalog.plpgsql"
 
@@ -380,6 +380,7 @@ ActiveRecord::Schema[8.0].define(version: 2025_12_02_140424) do
     t.integer "original_number_of_items"
     t.datetime "created_at", null: false
     t.datetime "updated_at", null: false
+    t.integer "item_count", default: 0, null: false
     t.index ["account_id"], name: "index_collections_on_account_id"
     t.index ["tag_id"], name: "index_collections_on_tag_id"
   end

spec/serializers/rest/base_collection_serializer_spec.rb

@@ -38,20 +38,4 @@ RSpec.describe REST::BaseCollectionSerializer do
         'updated_at' => match_api_datetime_format
       )
   end
-
-  describe 'Counting items' do
-    before do
-      Fabricate.times(2, :collection_item, collection:)
-    end
-
-    it 'can count items on demand' do
-      expect(subject['item_count']).to eq 2
-    end
-
-    it 'can use precalculated counts' do
-      collection.define_singleton_method :item_count, -> { 8 }
-
-      expect(subject['item_count']).to eq 8
-    end
-  end
 end