你好！
我当前处于版本 136a5456534c7e468b8052f865ce970a3bf375e9，在此版本中执行 multisite:migrate 会失败，错误信息如下：
I, [2020-05-07T06:57:21.288947 #1] INFO -- : > cd /var/www/discourse && sudo -Eu discourse bundle exec rake multisite:migrate
`/root` is not writable.
Bundler will use `/tmp/bundler20200507-300-1g5pu90300' as your home directory temporarily.
rake aborted!
NoMethodError: undefined method `call' for [:delete, RailsMultisite::Middleware]:Array
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/railties-6.0.3/lib/rails/configuration.rb:76:in `block in merge_into'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/railties-6.0.3/lib/rails/configuration.rb:75:in `each'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/railties-6.0.3/lib/rails/configuration.rb:75:in `merge_into'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/railties-6.0.3/lib/rails/engine.rb:512:in `block in app'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/railties-6.0.3/lib/rails/engine.rb:509:in `synchronize'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/railties-6.0.3/lib/rails/engine.rb:509:in `app'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/railties-6.0.3/lib/rails/application/finisher.rb:97:in `block in <module:Finisher>'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/railties-6.0.3/lib/rails/initializable.rb:32:in `instance_exec'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/railties-6.0.3/lib/rails/initializable.rb:32:in `run'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/railties-6.0.3/lib/rails/initializable.rb:61:in `block in run_initializers'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/railties-6.0.3/lib/rails/initializable.rb:60:in `run_initializers'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/railties-6.0.3/lib/rails/application.rb:363:in `initialize!'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/railties-6.0.3/lib/rails/railtie.rb:190:in `public_send'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/railties-6.0.3/lib/rails/railtie.rb:190:in `method_missing'
/var/www/discourse/config/environment.rb:7:in `<top (required)>'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/zeitwerk-2.3.0/lib/zeitwerk/kernel.rb:23:in `require'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/zeitwerk-2.3.0/lib/zeitwerk/kernel.rb:23:in `require'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/activesupport-6.0.3/lib/active_support/dependencies.rb:324:in `block in require'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/activesupport-6.0.3/lib/active_support/dependencies.rb:291:in `load_dependency'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/activesupport-6.0.3/lib/active_support/dependencies.rb:324:in `require'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/railties-6.0.3/lib/rails/application.rb:339:in `require_environment!'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/railties-6.0.3/lib/rails/application.rb:523:in `block in run_tasks_blocks'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/rake-13.0.1/exe/rake:27:in `<top (required)>'
/usr/local/bin/bundle:23:in `load'
/usr/local/bin/bundle:23:in `<main>'
Tasks: TOP => multisite:migrate => db:load_config => environment
(See full trace by running task with --trace)
I, [2020-05-07T06:57:26.316379 #1] INFO -- :
这似乎与 config/initializers/200-first_middlewares.rb 中的中间件操作有关。
回退到 136a5456534c7e468b8052f865ce970a3bf375e9 之前的版本（该版本将 Rails 升级到了 6.0.3），multisite:migrate 可以正常运行。我还有一个开发环境实例，其中我将 session_operations 替换为对 Rails.configuration.middleware 的 .delete 和 .unshift 操作，结果看起来正常。
（除了迁移 20141216112341_resolve_duplicate_group_names 失败的问题，它声称不存在的列 groups.automatic_membership_email_domains，但重试后该问题消失。）
我已经忘记了大部分曾经了解的 Rails 知识，所以我的分析可能完全错误！
