我正在阅读rails教程书，在第6章第199页我必须做一个rake db：migrate，但是我收到了这个错误：
耙子流产了！ nil的未定义方法“先决条件”：NilClass（通过使用--trace运行任务查看完整跟踪）
当我尝试使用rake db：migrate --trace时，我收到以下错误：
rake aborted!
undefined method `prerequisites' for nil:NilClass
/Users/jeanosorio/.rvm/gems/ruby-1.9.2-p290/gems/rspec-rails-2.0.1/lib/rspec/rails/tasks/rspec.rake:3:in `<top (required)>'
/Users/jeanosorio/.rvm/gems/ruby-1.9.2-p290/gems/rspec-rails-2.0.1/lib/rspec-rails.rb:10:in `load'
/Users/jeanosorio/.rvm/gems/ruby-1.9.2-p290/gems/rspec-rails-2.0.1/lib/rspec-rails.rb:10:in `block in <class:Railtie>'
/Users/jeanosorio/.rvm/gems/ruby-1.9.2-p290/gems/railties-3.1.1/lib/rails/railtie.rb:183:in `call'
/Users/jeanosorio/.rvm/gems/ruby-1.9.2-p290/gems/railties-3.1.1/lib/rails/railtie.rb:183:in `block in load_tasks'
/Users/jeanosorio/.rvm/gems/ruby-1.9.2-p290/gems/railties-3.1.1/lib/rails/railtie.rb:183:in `each'
/Users/jeanosorio/.rvm/gems/ruby-1.9.2-p290/gems/railties-3.1.1/lib/rails/railtie.rb:183:in `load_tasks'
/Users/jeanosorio/.rvm/gems/ruby-1.9.2-p290/gems/railties-3.1.1/lib/rails/engine.rb:396:in `block in load_tasks'
/Users/jeanosorio/.rvm/gems/ruby-1.9.2-p290/gems/railties-3.1.1/lib/rails/application/railties.rb:8:in `each'
/Users/jeanosorio/.rvm/gems/ruby-1.9.2-p290/gems/railties-3.1.1/lib/rails/application/railties.rb:8:in `all'
/Users/jeanosorio/.rvm/gems/ruby-1.9.2-p290/gems/railties-3.1.1/lib/rails/engine.rb:396:in `load_tasks'
/Users/jeanosorio/.rvm/gems/ruby-1.9.2-p290/gems/railties-3.1.1/lib/rails/application.rb:103:in `load_tasks'
/Users/jeanosorio/.rvm/gems/ruby-1.9.2-p290/gems/railties-3.1.1/lib/rails/railtie/configurable.rb:30:in `method_missing'
/Users/jeanosorio/ror/sample_app/Rakefile:7:in `<top (required)>'
/Users/jeanosorio/.rvm/gems/ruby-1.9.2-p290/gems/rake-0.9.2/lib/rake/rake_module.rb:25:in `load'
/Users/jeanosorio/.rvm/gems/ruby-1.9.2-p290/gems/rake-0.9.2/lib/rake/rake_module.rb:25:in `load_rakefile'
/Users/jeanosorio/.rvm/gems/ruby-1.9.2-p290/gems/rake-0.9.2/lib/rake/application.rb:495:in `raw_load_rakefile'
/Users/jeanosorio/.rvm/gems/ruby-1.9.2-p290/gems/rake-0.9.2/lib/rake/application.rb:78:in `block in load_rakefile'
/Users/jeanosorio/.rvm/gems/ruby-1.9.2-p290/gems/rake-0.9.2/lib/rake/application.rb:129:in `standard_exception_handling'
/Users/jeanosorio/.rvm/gems/ruby-1.9.2-p290/gems/rake-0.9.2/lib/rake/application.rb:77:in `load_rakefile'
/Users/jeanosorio/.rvm/gems/ruby-1.9.2-p290/gems/rake-0.9.2/lib/rake/application.rb:61:in `block in run'
/Users/jeanosorio/.rvm/gems/ruby-1.9.2-p290/gems/rake-0.9.2/lib/rake/application.rb:129:in `standard_exception_handling'
/Users/jeanosorio/.rvm/gems/ruby-1.9.2-p290/gems/rake-0.9.2/lib/rake/application.rb:59:in `run'
/Users/jeanosorio/.rvm/gems/ruby-1.9.2-p290/gems/rake-0.9.2/bin/rake:32:in `<top (required)>'
/Users/jeanosorio/.rvm/gems/ruby-1.9.2-p290/bin/rake:19:in `load'
/Users/jeanosorio/.rvm/gems/ruby-1.9.2-p290/bin/rake:19:in `<main>'
我尝试更新我的宝石，但我仍然收到最后一个错误。任何人都可以帮我解决这个问题。
答案 0 :(得分：0)
嘿，我有同样的问题，我相信这是Rails 3.1 changes that the book does not cover所致。我通过将我的GEMFILE更改为此来实现它：
source 'http://rubygems.org'
gem 'rails', '3.1.0'
gem 'gravatar_image_tag', '1.0.0.pre2'
gem 'will_paginate', '3.0.pre2'
gem 'sqlite3', '1.3.4'
# Asset template engines
gem 'sass-rails', "~> 3.1.0.rc"
gem 'coffee-script'
gem 'uglifier'
gem 'jquery-rails'
group :development do
gem 'rspec-rails', '2.6.1'
gem 'annotate', '2.4.0'
gem 'faker', '0.3.1'
end
group :test do
gem 'rspec-rails', '2.6.1'
gem 'webrat', '0.7.1'
gem 'spork', '0.9.0.rc5'
gem 'factory_girl_rails', '1.0'
end
将其复制并粘贴到您当前的GEMFILE上。接下来在终端中运行此命令
bundle install
现在你很高兴！