升级至Rails 6后，Concern中无法加载Mongoid的Boolean类型

我们在将应用从Ruby 2.7/Rails 5.2升级到Ruby 3.0/Rails 6.0时（因Ruby 3.0与Rails 5.2不兼容），遇到启动错误：uninitialized constant UserFields::Boolean (NameError)。错误出现在把User模型的字段拆分到UserFields concern中时，直接在User模型里定义Boolean类型字段则正常运行，原代码在旧环境下已稳定运行多年。

代码示例 #

User模型代码 #

class User
  include Mongoid::Document
  include Mongoid::Timestamps
  #field :force_password_change, type: Boolean, default: false
  include UserFields
  ....
end

UserFields Concern代码 #

module UserFields
  extend ActiveSupport::Concern

  included do
    ## Database authenticatable
    field :email,              :type => String, :default => ""
    field :force_password_change, type: Boolean, default: false  ### <<<< ERROR HERE
  end
end

错误追踪栈 #

! Unable to load application: NameError: uninitialized constant UserFields::Boolean
2024-01-10 15:22:22 bundler: failed to load command: puma (/app/bundle_cache/ruby/3.0.0/bin/puma)
2024-01-10 15:22:22 /app/app/models/concerns/user_fields.rb:8:in `block in <module:UserFields>': uninitialized constant UserFields::Boolean (NameError)
2024-01-10 15:22:22     from /app/bundle_cache/ruby/3.0.0/gems/activesupport-6.0.6.1/lib/active_support/concern.rb:122:in `class_eval'
2024-01-10 15:22:22     from /app/bundle_cache/ruby/3.0.0/gems/activesupport-6.0.6.1/lib/active_support/concern.rb:122:in `append_features'
2024-01-10 15:22:22     from /app/app/models/user.rb:9:in `include'
2024-01-10 15:22:22     from /app/app/models/user.rb:9:in `<class:User>'
2024-01-10 15:22:22     from /app/app/models/user.rb:4:in `<main>'
2024-01-10 15:22:22     from /app/bundle_cache/ruby/3.0.0/gems/bootsnap-1.17.0/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:32:in `require'
2024-01-10 15:22:22     from /app/bundle_cache/ruby/3.0.0/gems/bootsnap-1.17.0/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:32:in `require'
2024-01-10 15:22:22     from /app/bundle_cache/ruby/3.0.0/gems/zeitwerk-2.6.12/lib/zeitwerk/kernel.rb:30:in `require'
2024-01-10 15:22:22     from /app/bundle_cache/ruby/3.0.0/gems/activesupport-6.0.6.1/lib/active_support/inflector/methods.rb:282:in `const_get'
2024-01-10 15:22:22     from /app/bundle_cache/ruby/3.0.0/gems/activesupport-6.0.6.1/lib/active_support/inflector/methods.rb:282:in `block in constantize'
2024-01-10 15:22:22     from /app/bundle_cache/ruby/3.0.0/gems/activesupport-6.0.6.1/lib/active_support/inflector/methods.rb:280:in `each'
2024-01-10 15:22:22     from /app/bundle_cache/ruby/3.0.0/gems/activesupport-6.0.6.1/lib/active_support/inflector/methods.rb:280:in `inject'
2024-01-10 15:22:22     from /app/bundle_cache/ruby/3.0.0/gems/activesupport-6.0.6.1/lib/active_support/inflector/methods.rb:280:in `constantize'
2024-01-10 15:22:22     from /app/bundle_cache/ruby/3.0.0/gems/activesupport-6.0.6.1/lib/active_support/dependencies/zeitwerk_integration.rb:19:in `constantize'
2024-01-10 15:22:22     from /app/bundle_cache/ruby/3.0.0/gems/devise-4.9.3/lib/devise.rb:325:in `get'
2024-01-10 15:22:22     from /app/bundle_cache/ruby/3.0.0/gems/devise-4.9.3/lib/devise/mapping.rb:83:in `to'
2024-01-10 15:22:22     from /app/bundle_cache/ruby/3.0.0/gems/devise-4.9.3/lib/devise/mapping.rb:78:in `modules'
2024-01-10 15:22:22     from /app/bundle_cache/ruby/3.0.0/gems/devise-4.9.3/lib/devise/mapping.rb:95:in `routes'
2024-01-10 15:22:22     from /app/bundle_cache/ruby/3.0.0/gems/devise-4.9.3/lib/devise/mapping.rb:162:in `default_used_route'
2024-01-10 15:22:22     from /app/bundle_cache/ruby/3.0.0/gems/devise-4.9.3/lib/devise/mapping.rb:72:in `initialize'
2024-01-10 15:22:22     from /app/bundle_cache/ruby/3.0.0/gems/devise-4.9.3/lib/devise.rb:361:in `new'
2024-01-10 15:22:22     from /app/bundle_cache/ruby/3.0.0/gems/devise-4.9.3/lib/devise.rb:361:in `add_mapping'
2024-01-10 15:22:22     from /app/bundle_cache/ruby/3.0.0/gems/devise-4.9.3/lib/devise/rails/routes.rb:243:in `block in devise_for'
2024-01-10 15:22:22     from /app/bundle_cache/ruby/3.0.0/gems/devise-4.9.3/lib/devise/rails/routes.rb:242:in `each'
2024-01-10 15:22:22     from /app/bundle_cache/ruby/3.0.0/gems/devise-4.9.3/lib/devise/rails/routes.rb:242:in `devise_for'
2024-01-10 15:22:22     from /app/config/routes.rb:264:in `block in <main>'
2024-01-10 15:22:22     from /app/bundle_cache/ruby/3.0.0/gems/actionpack-6.0.6.1/lib/action_dispatch/routing/route_set.rb:426:in `instance_exec'
2024-01-10 15:22:22     from /app/bundle_cache/ruby/3.0.0/gems/actionpack-6.0.6.1/lib/action_dispatch/routing/route_set.rb:426:in `eval_block'
2024-01-10 15:22:22     from /app/bundle_cache/ruby/3.0.0/gems/actionpack-6.0.6.1/lib/action_dispatch/routing/route_set.rb:408:in `draw'
2024-01-10 15:22:22     from /app/config/routes.rb:1:in `<main>'

解决方案 #

方案1：使用根命名空间常量 #

在Concern里把Boolean替换成::Boolean，强制从根命名空间查找常量：

field :force_password_change, type: ::Boolean, default: false

方案2：显式指定Mongoid类型 #

直接使用Mongoid::Boolean或者引入Mongoid的类型模块：

# 直接指定Mongoid类型
field :force_password_change, type: Mongoid::Boolean, default: false

# 或者在模块顶部引入类型模块
module UserFields
  extend ActiveSupport::Concern
  include Mongoid::Fields::Types

  included do
    field :force_password_change, type: Boolean, default: false
  end
end

方案3：使用符号替代常量 #

Mongoid支持用:boolean符号指定字段类型，完全避开常量查找问题：

field :force_password_change, type: :boolean, default: false

错误原因 #

Rails 6.0默认使用Zeitwerk加载器，常量查找规则比旧版Autoloader更严格。在Concern的included块中，常量会优先在当前模块（UserFields）的命名空间下查找，而旧环境中会自动回退到根命名空间。加上Devise在路由加载时提前触发了User模型的加载，导致常量查找时机与旧环境不同，最终触发错误。

内容的提问来源于stack exchange，提问作者timsegraves