mastodon/db/migrate/20170330021336_add_counter_caches.rb

class AddCounterCaches < ActiveRecord::Migration[5.0]
  def change
    add_column :statuses, :favourites_count, :integer, null: false, default: 0
    add_column :statuses, :reblogs_count, :integer, null: false, default: 0
    add_column :accounts, :statuses_count, :integer, null: false, default: 0
    add_column :accounts, :followers_count, :integer, null: false, default: 0
    add_column :accounts, :following_count, :integer, null: false, default: 0
  end
end

# To make the new fields contain correct data:
# update statuses set favourites_count = (select count(*) from favourites where favourites.status_id = statuses.id), reblogs_count = (select count(*) from statuses as reblogs where reblogs.reblog_of_id = statuses.id);
# update accounts set statuses_count = (select count(*) from statuses where account_id = accounts.id), followers_count = (select count(*) from follows where target_account_id = accounts.id), following_count = (select count(*) from follows where account_id = accounts.id);