class ChangeSubscribedUsersToSet < ActiveRecord::Migration[4.2]
  def up
    contacts = connection.select_all %(
      SELECT id, subscribed_users
      FROM contacts
      WHERE subscribed_users IS NOT NULL
        )

    puts "Converting #{contacts.size} subscribed_users arrays into sets..." unless contacts.empty?

    # Run as one atomic action.
    ActiveRecord::Base.transaction do
      contacts.each do |contact|
        subscribed_users_set = Set.new(YAML.load(contact["subscribed_users"]))

        connection.execute %(
          UPDATE contacts
          SET subscribed_users = '#{subscribed_users_set.to_yaml}'
          WHERE id = #{contact['id']}
                )
      end
    end
  end
end