升级Rails 4应用执行rails app:update报错的解决求助

问题原因 #

app:update是Rails 5及更高版本才新增的任务，Rails 4中不存在该任务，因此执行命令会提示找不到任务。

解决方案 #

1. 使用Rails 4对应升级命令
   替换为Rails 4支持的升级命令即可：

   rake rails:update
   

   该命令会完成Rails配置文件、脚本等的更新，功能和Rails 5+的app:update一致。

2. 确认依赖版本兼容性
   检查Gemfile中rake版本是否与Rails 4兼容，Rails 4.x通常搭配rake 10.x或11.x版本，当前使用的rake 10.1.1是兼容的。若后续出现依赖问题，可尝试更新rake到兼容版本：

   # 在Gemfile中修改
   gem 'rake', '~> 10.5.0'
   

   执行bundle install完成依赖更新。

升级额外注意事项 #

• 执行升级前务必备份代码或提交当前修改，避免文件冲突导致数据丢失。
• 先运行bundle update rails将Rails升级到4.x系列最新稳定版，再执行升级命令。
• 命令执行中若提示文件冲突，根据实际需求选择保留原有文件或替换为新版本模板。

完整报错堆栈 #

Don't know how to build task 'app:update'
/Users/akshar/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/rake-10.1.1/lib/rake/task_manager.rb:49:in `[]'
/Users/akshar/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/rake-10.1.1/lib/rake/application.rb:148:in `invoke_task'
/Users/akshar/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/rake-10.1.1/lib/rake/application.rb:106:in `block (2 levels) in top_level'
/Users/akshar/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/rake-10.1.1/lib/rake/application.rb:106:in `each'
/Users/akshar/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/rake-10.1.1/lib/rake/application.rb:106:in `block in top_level'
/Users/akshar/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/rake-10.1.1/lib/rake/application.rb:115:in `run_with_threads'
/Users/akshar/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/rake-10.1.1/lib/rake/application.rb:100:in `top_level'
/Users/akshar/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/rake-10.1.1/lib/rake/application.rb:78:in `block in run'
/Users/akshar/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/rake-10.1.1/lib/rake/application.rb:165:in `standard_exception_handling'
/Users/akshar/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/rake-10.1.1/lib/rake/application.rb:75:in `run'
/Users/akshar/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/rake-10.1.1/bin/rake:33:in `<top (required)>'
/Users/akshar/.rbenv/versions/2.6.10/bin/rake:23:in `load'
/Users/akshar/.rbenv/versions/2.6.10/bin/rake:23:in `<top (required)>'
/Users/akshar/.rbenv/versions/2.6.10/lib/ruby/2.6.0/bundler/cli/exec.rb:74:in `load'
/Users/akshar/.rbenv/versions/2.6.10/lib/ruby/2.6.0/bundler/cli/exec.rb:74:in `kernel_load'
/Users/akshar/.rbenv/versions/2.6.10/lib/ruby/2.6.0/bundler/cli/exec.rb:28:in `run'
/Users/akshar/.rbenv/versions/2.6.10/lib/ruby/2.6.0/bundler/cli.rb:463:in `exec'
/Users/akshar/.rbenv/versions/2.6.10/lib/ruby/2.6.0/bundler/vendor/thor/lib/thor/command.rb:27:in `run'
/Users/akshar/.rbenv/versions/2.6.10/lib/ruby/2.6.0/bundler/vendor/thor/lib/thor/invocation.rb:126:in `invoke_command'
/Users/akshar/.rbenv/versions/2.6.10/lib/ruby/2.6.0/bundler/vendor/thor/lib/thor.rb:387:in `dispatch'
/Users/akshar/.rbenv/versions/2.6.10/lib/ruby/2.6.0/bundler/cli.rb:27:in `dispatch'
/Users/akshar/.rbenv/versions/2.6.10/lib/ruby/2.6.0/bundler/vendor/thor/lib/thor/base.rb:466:in `start'
/Users/akshar/.rbenv/versions/2.6.10/lib/ruby/2.6.0/bundler/cli.rb:18:in `start'
/Users/akshar/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/bundler-1.17.2/exe/bundle:30:in `block in <top (required)>'
/Users/akshar/.rbenv/versions/2.6.10/lib/ruby/2.6.0/bundler/friendly_errors.rb:124:in `with_friendly_errors'
/Users/akshar/.rbenv/versions/2.6.10/lib/ruby/gems/2.6.0/gems/bundler-1.17.2/exe/bundle:22:in `<top (required)>'
/Users/akshar/.rbenv/versions/2.6.10/bin/bundle:23:in `load'
/Users/akshar/.rbenv/versions/2.6.10/bin/bundle:23:in `<main>'

内容的提问来源于stack exchange，提问作者opensource-developer