group member count optimization

app/models/group.rb

@@ -15,6 +15,7 @@
 #  is_archived              :boolean          default(FALSE), not null
 #  created_at               :datetime         not null
 #  updated_at               :datetime         not null
+#  member_count             :integer          default(0)
 #
 
 class Group < ApplicationRecord

app/models/group_account.rb

@@ -21,10 +21,20 @@ class GroupAccount < ApplicationRecord
   validates :account_id, uniqueness: { scope: :group_id }
 
   after_commit :remove_relationship_cache
+  after_create :increment_member_count
+  after_destroy :decrement_member_count
 
   private
 
   def remove_relationship_cache
     Rails.cache.delete("relationship:#{account_id}:group#{group_id}")
   end
+
+  def increment_member_count
+    group&.increment!(:member_count)
+  end
+
+  def decrement_member_count
+    group&.decrement!(:member_count)
+  end
 end

app/serializers/rest/group_serializer.rb

@@ -22,8 +22,4 @@ class REST::GroupSerializer < ActiveModel::Serializer
 
     full_asset_url(object.cover_image.url)
   end
-
-  def member_count
-    object.accounts.count
-  end
 end

db/migrate/20190722003541_add_member_count_to_groups.rb

@@ -0,0 +1,10 @@
+class AddMemberCountToGroups < ActiveRecord::Migration[5.2]
+  def up
+    add_column :groups, :member_count, :integer
+    change_column_default :groups, :member_count, 0
+  end
+
+  def down
+    remove_column :groups, :member_count
+  end
+end

db/migrate/20190722003649_backfill_add_member_count_to_groups.rb

@@ -0,0 +1,10 @@
+class BackfillAddMemberCountToGroups < ActiveRecord::Migration[5.2]
+  disable_ddl_transaction!
+
+  def change
+    Group.in_batches do |relation|
+      relation.update_all member_count: 0
+      sleep(0.1)
+    end
+  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.define(version: 2019_07_21_234917) do
+ActiveRecord::Schema.define(version: 2019_07_22_003649) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -358,6 +358,7 @@ ActiveRecord::Schema.define(version: 2019_07_21_234917) do
     t.boolean "is_archived", default: false, null: false
     t.datetime "created_at", null: false
     t.datetime "updated_at", null: false
+    t.integer "member_count", default: 0
     t.index ["account_id"], name: "index_groups_on_account_id"
   end
 

lib/tasks/fix_group_member_counts.rake

@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+
+task fix_group_member_counts: 'gabsocial:fix-group-member-counts'
+
+namespace :gabsocial do
+  desc 'Re-compute group member counts'
+  task :fix_group_member_counts => :environment do
+    Group.select(:id).all.each do |group|
+      group.update_column(:member_count, group.accounts.count)
+    end
+  end
+end