它是做什么的“捆绑exec rake db：migrate”？
根据我的研究，bundle exec负责在bundle的上下文中执行命令。
无论如何，我还不太明白它对于命令rake db:migrate什么不同rake db:migrate与bundle exec rake db:migrate 。
例如在我的情况下，我执行了第一个命令，我得到的是以下错误：
$ rake db:migrate (in c:/rails/rails_projects/soccerweb) rake aborted! uninitialized constant Rake::DSL c:/Ruby192/lib/ruby/1.9.1/rake.rb:2482:in `const_missing' c:/Ruby192/lib/ruby/gems/1.9.1/gems/rake-0.9.2.2/lib/rake/tasklib.rb:8:in `' c:/Ruby192/lib/ruby/gems/1.9.1/gems/rake-0.9.2.2/lib/rake/tasklib.rb:6:in `' c:/Ruby192/lib/ruby/gems/1.9.1/gems/rake-0.9.2.2/lib/rake/tasklib.rb:3:in `' c:/Ruby192/lib/ruby/gems/1.9.1/gems/rake-0.9.2.2/lib/rake/testtask.rb:4:in `requ ire' c:/Ruby192/lib/ruby/gems/1.9.1/gems/rake-0.9.2.2/lib/rake/testtask.rb:4:in `' c:/Ruby192/lib/ruby/gems/1.9.1/gems/railties-3.1.1/lib/rails/test_unit/testing.r ake:2:in `require' c:/Ruby192/lib/ruby/gems/1.9.1/gems/railties-3.1.1/lib/rails/test_unit/testing.r ake:2:in `' c:/Ruby192/lib/ruby/gems/1.9.1/gems/railties-3.1.1/lib/rails/test_unit/railtie.r b:12:in `load' c:/Ruby192/lib/ruby/gems/1.9.1/gems/railties-3.1.1/lib/rails/test_unit/railtie.r b:12:in `block in ' c:/Ruby192/lib/ruby/gems/1.9.1/gems/railties-3.1.1/lib/rails/railtie.rb:183:in ` call' c:/Ruby192/lib/ruby/gems/1.9.1/gems/railties-3.1.1/lib/rails/railtie.rb:183:in ` block in load_tasks' c:/Ruby192/lib/ruby/gems/1.9.1/gems/railties-3.1.1/lib/rails/railtie.rb:183:in ` each' c:/Ruby192/lib/ruby/gems/1.9.1/gems/railties-3.1.1/lib/rails/railtie.rb:183:in ` load_tasks' c:/Ruby192/lib/ruby/gems/1.9.1/gems/railties-3.1.1/lib/rails/engine.rb:396:in `b lock in load_tasks' c:/Ruby192/lib/ruby/gems/1.9.1/gems/railties-3.1.1/lib/rails/application/railtie s.rb:8:in `each' c:/Ruby192/lib/ruby/gems/1.9.1/gems/railties-3.1.1/lib/rails/application/railtie s.rb:8:in `all' c:/Ruby192/lib/ruby/gems/1.9.1/gems/railties-3.1.1/lib/rails/engine.rb:396:in `l oad_tasks' c:/Ruby192/lib/ruby/gems/1.9.1/gems/railties-3.1.1/lib/rails/application.rb:103: in `load_tasks' c:/Ruby192/lib/ruby/gems/1.9.1/gems/railties-3.1.1/lib/rails/railtie/configurabl e.rb:30:in `method_missing' c:/rails/rails_projects/soccerweb/Rakefile:7:in `' c:/Ruby192/lib/ruby/1.9.1/rake.rb:2373:in `load' c:/Ruby192/lib/ruby/1.9.1/rake.rb:2373:in `raw_load_rakefile' c:/Ruby192/lib/ruby/1.9.1/rake.rb:2007:in `block in load_rakefile' c:/Ruby192/lib/ruby/1.9.1/rake.rb:2058:in `standard_exception_handling' c:/Ruby192/lib/ruby/1.9.1/rake.rb:2006:in `load_rakefile' c:/Ruby192/lib/ruby/1.9.1/rake.rb:1991:in `run' c:/Ruby192/bin/rake:31:in `'
但是当我在“rake db：migrate”命令之前使用“bundle exec”时，一切都优雅地为我工作！
谁能对我有所了解？
bundle exec rake运行你在Gemfile指定的rake版本。 rake的默认版本（可以在$PATH找到的版本）可能与bundle exec rake执行的版本不同。
您遇到的问题已在单独的问题中介绍 。 您的bundle exec rake最有可能运行rake 0.8.7并且来自$PATH rake是受此问题影响的较新版本。
引自man bundle exec ：
bundle exec对shell环境进行了一些更改，然后执行完全指定的命令。
- （……）
- 将包含可执行文件的目录（例如rails，rspec，rackup）放在$ PATH上
Bundler为您解决gem依赖问题。 您可以在基本原理页面中找到有关它的最佳解释。