Populate a cached counter field via a single SQL query.

counter_backfill_migration.rb

# Credit to Mike Perham for this gem of an idea
# http://www.mikeperham.com/2007/12/17/creating-a-counter_cache-column/
class RailsMigration < ActiveRecord::Migration
  def self.up
    add_column :media, :followers_count, :integer, :default => 0, :null => false

    # Populate the media follower counter cache in one fell swoop.
    sql = "update media, (select operable_id, count(*) as the_count from follow_operations "\
      "group by operable_id) as follows set media.followers_count = follows.the_count "\
      "where media.id = follows.operable_id;"
    ActiveRecord::Base.connection.execute(sql);
  end

  def self.down
    remove_column :media, :followers_count
  end
end