我正在关注“Heroku上的Ruby入门”，但在尝试使用
本地运行演示应用时$ bundle exec rake db:create db:migrate
我得到了
sam@samoliver:~/ruby-getting-started$ bundle exec rake db:create db:migrate --trace
rake aborted!
Bundler::GemRequireError: There was an error while trying to load the gem 'uglifier'.
/home/sam/.rvm/gems/ruby-2.2.3/gems/bundler-1.11.0/lib/bundler/runtime.rb:80:in `rescue in block (2 levels) in require'
/home/sam/.rvm/gems/ruby-2.2.3/gems/bundler-1.11.0/lib/bundler/runtime.rb:76:in `block (2 levels) in require'
/home/sam/.rvm/gems/ruby-2.2.3/gems/bundler-1.11.0/lib/bundler/runtime.rb:72:in `each'
/home/sam/.rvm/gems/ruby-2.2.3/gems/bundler-1.11.0/lib/bundler/runtime.rb:72:in `block in require'
/home/sam/.rvm/gems/ruby-2.2.3/gems/bundler-1.11.0/lib/bundler/runtime.rb:61:in `each'
/home/sam/.rvm/gems/ruby-2.2.3/gems/bundler-1.11.0/lib/bundler/runtime.rb:61:in `require'
/home/sam/.rvm/gems/ruby-2.2.3/gems/bundler-1.11.0/lib/bundler.rb:99:in `require'
/home/sam/ruby-getting-started/config/application.rb:7:in `<top (required)>'
/home/sam/ruby-getting-started/Rakefile:4:in `require'
/home/sam/ruby-getting-started/Rakefile:4:in `<top (required)>'
/home/sam/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rake/rake_module.rb:28:in `load'
/home/sam/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rake/rake_module.rb:28:in `load_rakefile'
/home/sam/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rake/application.rb:689:in `raw_load_rakefile'
/home/sam/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rake/application.rb:94:in `block in load_rakefile'
/home/sam/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rake/application.rb:176:in `standard_exception_handling'
/home/sam/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rake/application.rb:93:in `load_rakefile'
/home/sam/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rake/application.rb:77:in `block in run'
/home/sam/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rake/application.rb:176:in `standard_exception_handling'
/home/sam/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rake/application.rb:75:in `run'
/home/sam/.rvm/rubies/ruby-2.2.3/bin/rake:33:in `<main>'
ExecJS::RuntimeUnavailable: Could not find a JavaScript runtime. See https://github.com/sstephenson/execjs for a list of available runtimes.
/home/sam/.rvm/gems/ruby-2.2.3/gems/execjs-2.2.2/lib/execjs/runtimes.rb:51:in `autodetect'
/home/sam/.rvm/gems/ruby-2.2.3/gems/execjs-2.2.2/lib/execjs.rb:5:in `<module:ExecJS>'
/home/sam/.rvm/gems/ruby-2.2.3/gems/execjs-2.2.2/lib/execjs.rb:4:in `<top (required)>'
/home/sam/.rvm/gems/ruby-2.2.3/gems/uglifier-2.5.3/lib/uglifier.rb:3:in `require'
/home/sam/.rvm/gems/ruby-2.2.3/gems/uglifier-2.5.3/lib/uglifier.rb:3:in `<top (required)>'
/home/sam/.rvm/gems/ruby-2.2.3/gems/bundler-1.11.0/lib/bundler/runtime.rb:77:in `require'
/home/sam/.rvm/gems/ruby-2.2.3/gems/bundler-1.11.0/lib/bundler/runtime.rb:77:in `block (2 levels) in require'
/home/sam/.rvm/gems/ruby-2.2.3/gems/bundler-1.11.0/lib/bundler/runtime.rb:72:in `each'
/home/sam/.rvm/gems/ruby-2.2.3/gems/bundler-1.11.0/lib/bundler/runtime.rb:72:in `block in require'
/home/sam/.rvm/gems/ruby-2.2.3/gems/bundler-1.11.0/lib/bundler/runtime.rb:61:in `each'
/home/sam/.rvm/gems/ruby-2.2.3/gems/bundler-1.11.0/lib/bundler/runtime.rb:61:in `require'
/home/sam/.rvm/gems/ruby-2.2.3/gems/bundler-1.11.0/lib/bundler.rb:99:in `require'
/home/sam/ruby-getting-started/config/application.rb:7:in `<top (required)>'
/home/sam/ruby-getting-started/Rakefile:4:in `require'
/home/sam/ruby-getting-started/Rakefile:4:in `<top (required)>'
/home/sam/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rake/rake_module.rb:28:in `load'
/home/sam/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rake/rake_module.rb:28:in `load_rakefile'
/home/sam/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rake/application.rb:689:in `raw_load_rakefile'
/home/sam/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rake/application.rb:94:in `block in load_rakefile'
/home/sam/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rake/application.rb:176:in `standard_exception_handling'
/home/sam/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rake/application.rb:93:in `load_rakefile'
/home/sam/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rake/application.rb:77:in `block in run'
/home/sam/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rake/application.rb:176:in `standard_exception_handling'
/home/sam/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rake/application.rb:75:in `run'
/home/sam/.rvm/rubies/ruby-2.2.3/bin/rake:33:in `<main>'
sam@samoliver:~/ruby-getting-started$
我尝试删除并再次添加'uglifier'（2.5.3）无效，我确实安装了'therubyracer'，所以我不明白ExecJS抱怨什么。
的Gemfile
source 'https://rubygems.org'
ruby '2.2.3'
# Bundle edge Rails instead: gem 'rails', github: 'rails/rails'
gem 'rails', '4.2.3'
# Use postgresql as the database for Active Record
gem 'pg'
gem 'rails_12factor', group: :production
# Use SCSS for stylesheets
gem 'sass-rails', '~> 5.0.3'
# Use Uglifier as compressor for JavaScript assets
gem 'uglifier', '>= 1.3.0'
# Use CoffeeScript for .js.coffee assets and views
gem 'coffee-rails', '~> 4.0.0'
# See https://github.com/sstephenson/execjs#readme for more supported runtimes
# gem 'therubyracer', platforms: :ruby
# Use jquery as the JavaScript library
gem 'jquery-rails'
# Turbolinks makes following links in your web application faster. Read more: https://github.com/rails/turbolinks
gem 'turbolinks'
# Build JSON APIs with ease. Read more: https://github.com/rails/jbuilder
gem 'jbuilder', '~> 2.0'
# bundle exec rake doc:rails generates the API under doc/api.
gem 'sdoc', '~> 0.4.0', group: :doc
# Spring speeds up development by keeping your application running in the background. Read more: https://github.com/rails/spring
gem 'spring', group: :development
# Use ActiveModel has_secure_password
# gem 'bcrypt', '~> 3.1.7'
gem 'puma'
# Use Capistrano for deployment
# gem 'capistrano-rails', group: :development
# Use debugger
# gem 'debugger', group: [:development, :test]
答案 0 :(得分：46)
安装nodejs
apt-get install nodejs
答案 1 :(得分：11)
取消注释Gemfile中的以下行
#gem 'therubyracer', platforms: :ruby
在终端
中运行bundle install
答案 2 :(得分：1)
Uglifier是JavaScript压缩器，因此需要javascript运行时尝试安装nodejs
运行此命令在Ubuntu中安装NodeJS
sudo apt-get install nodejs
或
brew install nodejs
如果你在OSX上。
答案 3 :(得分：0)
我遇到了同样的问题。我必须进入我的根文件夹（在ubuntu上使用
sudo su)
从那里使用apt-get install nodejs，然后退出根并运行bundle install，那么您应该可以运行rake db:migrate
答案 4 :(得分：0)
试试这个：
gem update --system 1.4.2