[Rails][CodeReading] Rails::Initializer (33) initialize_database_middleware

Initializer.process で33番目に呼ばれる initialize_database_middleware メソッド。

    def initialize_database_middleware
      if configuration.frameworks.include?(:active_record)
        if configuration.frameworks.include?(:action_controller) &&
            ActionController::Base.session_store == ActiveRecord::SessionStore
          configuration.middleware.insert_before :"ActiveRecord::SessionStore", ActiveRecord::ConnectionAdapters::ConnectionManagement
          configuration.middleware.insert_before :"ActiveRecord::SessionStore", ActiveRecord::QueryCache
        else
          configuration.middleware.use ActiveRecord::ConnectionAdapters::ConnectionManagement
          configuration.middleware.use ActiveRecord::QueryCache
        end
      end
    end

SessionStoreの準備などをしているらしい？

[Rails][CodeReading] Railsの初期化コードを読む (イントロ＆目次)