Heroku部署Rails应用报Could not detect rake tasks错误求助

问题现象 #

• 推送Rails项目代码到Heroku平台时部署失败，bundle install阶段执行完毕后抛出异常
• 已尝试以下常见排查方案均未解决问题：
  • 在Gemfile中显式声明rake依赖
  • 为Gemfile.lock补充linux、ruby平台适配配置
  • 降级bundler版本安装依赖
  • 升级Heroku buildpack版本
  • 升级Heroku stack版本

报错日志 #

remote:  !     Could not detect rake tasks
remote:  !     ensure you can run `$ bundle exec rake -P` against your app
remote:  !     and using the production group of your Gemfile.
remote:  !     rake aborted!
remote:  !     LoadError: cannot load such file -- rspec/core
remote:  !     /tmp/build_7d354c0f/vendor/bundle/ruby/3.1.0/gems/bootsnap-1.12.0/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:15:in `require'
remote:  !     /tmp/build_7d354c0f/vendor/bundle/ruby/3.1.0/gems/bootsnap-1.12.0/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:15:in `require'
remote:  !     /tmp/build_7d354c0f/vendor/bundle/ruby/3.1.0/gems/rswag-specs-2.5.1/lib/rswag/specs.rb:3:in `<main>'
remote:  !     /tmp/build_7d354c0f/vendor/bundle/ruby/3.1.0/gems/bootsnap-1.12.0/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:30:in `require'
remote:  !     /tmp/build_7d354c0f/vendor/bundle/ruby/3.1.0/gems/bootsnap-1.12.0/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:30:in `require'
remote:  !     /tmp/build_7d354c0f/vendor/bundle/ruby/3.1.0/gems/rswag-2.5.1/lib/rswag.rb:1:in `<main>'
remote:  !     /tmp/build_7d354c0f/vendor/bundle/ruby/3.1.0/gems/bootsnap-1.12.0/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:30:in `require'
remote:  !     /tmp/build_7d354c0f/vendor/bundle/ruby/3.1.0/gems/bootsnap-1.12.0/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:30:in `require'
remote:  !     /tmp/build_7d354c0f/vendor/bundle/ruby/3.1.0/gems/bundler-2.3.10/lib/bundler/runtime.rb:60:in `block (2 levels) in require'
remote:  !     /tmp/build_7d354c0f/vendor/bundle/ruby/3.1.0/gems/bundler-2.3.10/lib/bundler/runtime.rb:55:in `each'
remote:  !     /tmp/build_7d354c0f/vendor/bundle/ruby/3.1.0/gems/bundler-2.3.10/lib/bundler/runtime.rb:55:in `block in require'
remote:  !     /tmp/build_7d354c0f/vendor/bundle/ruby/3.1.0/gems/bundler-2.3.10/lib/bundler/runtime.rb:44:in `each'
remote:  !     /tmp/build_7d354c0f/vendor/bundle/ruby/3.1.0/gems/bundler-2.3.10/lib/bundler/runtime.rb:44:in `require'
remote:  !     /tmp/build_7d354c0f/vendor/bundle/ruby/3.1.0/gems/bundler-2.3.10/lib/bundler.rb:176:in `require'
remote:  !     /tmp/build_7d354c0f/config/application.rb:7:in `<main>'
remote:  !     /tmp/build_7d354c0f/vendor/bundle/ruby/3.1.0/gems/bootsnap-1.12.0/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:30:in `require'
remote:  !     /tmp/build_7d354c0f/vendor/bundle/ruby/3.1.0/gems/bootsnap-1.12.0/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:30:in `require'
remote:  !     /tmp/build_7d354c0f/Rakefile:4:in `<main>'
remote:  !     /tmp/build_7d354c0f/vendor/bundle/ruby/3.1.0/gems/bootsnap-1.12.0/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:39:in `load'
remote:  !     /tmp/build_7d354c0f/vendor/bundle/ruby/3.1.0/gems/bootsnap-1.12.0/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:39:in `load'
remote:  !     /tmp/build_7d354c0f/vendor/bundle/ruby/3.1.0/gems/rake-13.0.6/lib/rake/rake_module.rb:29:in `load_rakefile'
remote:  !     /tmp/build_7d354c0f/vendor/bundle/ruby/3.1.0/gems/rake-13.0.6/lib/rake/application.rb:710:in `raw_load_rakefile'
remote:  !     /tmp/build_7d354c0f/vendor/bundle/ruby/3.1.0/gems/rake-13.0.6/lib/rake/application.rb:104:in `block in load_rakefile'
remote:  !     /tmp/build_7d354c0f/vendor/bundle/ruby/3.1.0/gems/rake-13.0.6/lib/rake/application.rb:186:in `standard_exception_handling'
remote:  !     /tmp/build_7d354c0f/vendor/bundle/ruby/3.1.0/gems/rake-13.0.6/lib/rake/application.rb:103:in `load_rakefile'
remote:  !     /tmp/build_7d354c0f/vendor/bundle/ruby/3.1.0/gems/rake-13.0.6/lib/rake/application.rb:82:in `block in run'
remote:  !     /tmp/build_7d354c0f/vendor/bundle/ruby/3.1.0/gems/rake-13.0.6/lib/rake/application.rb:186:in `standard_exception_handling'
remote:  !     /tmp/build_7d354c0f/vendor/bundle/ruby/3.1.0/gems/rake-13.0.6/lib/rake/application.rb:80:in 'run'
remote:  !     /tmp/build_7d354c0f/bin/rake:4:in `<main>'
remote:  !

根因分析 #

报错核心是生产环境加载了仅测试环境所需的依赖：
从调用栈可以定位到，生产环境启动时Bundler自动加载了rswag gem，其依赖的rswag-specs组件强依赖rspec/core，但rspec系列gem默认放在test分组下，Heroku生产环境部署默认不会安装test、development分组的依赖，最终触发LoadError，导致rake任务检测失败。

修复方案 #

1. 打开项目config/application.rb文件，定位到Bundler加载依赖的配置行，错误写法会无差别加载全部分组的gem：

# 错误配置：会尝试加载所有分组gem，包括未在生产环境安装的test分组依赖
Bundler.require(*Rails.groups)

2. 修改配置，明确指定按当前运行环境加载对应分组的gem，生产环境仅加载default和production分组依赖：

# 正确配置：不会尝试加载test/development分组的未安装依赖
Bundler.require(:default, Rails.env)

3. 如果业务需要在生产环境加载rswag能力（比如对外暴露API文档页面），则调整Gemfile，把rswag相关依赖移出test分组，同时补充rspec-core到生产依赖中：

# Gemfile调整示例
gem 'rswag'
gem 'rswag-specs'
gem 'rspec-core'

4. 本地执行bundle install更新Gemfile.lock，提交变更后重新推送到Heroku即可。

本地预验证可以执行RAILS_ENV=production bundle exec rake -P，命令无报错即代表生产环境rake任务加载逻辑正常，不会触发Heroku的检测拦截。

内容的提问来源于stack exchange，提问作者Marius