title: rake assets:precompile RAILS_ENV=production not working as required
body: <p>I am trying to precompile assets using the command <code>rake assets:precompile RAILS_ENV=production</code>, but I always get the error below.</p>
  
  <pre><code>** Invoke assets:precompile (first_time)
      ** Execute assets:precompile
      /usr/bin/ruby /usr/bin/rake assets:precompile:all RAILS_ENV=production RAILS_GROUPS=assets --trace
      ** Invoke assets:precompile:all (first_time)
      ** Execute assets:precompile:all
      ** Invoke assets:precompile:primary (first_time)
      ** Invoke assets:environment (first_time)
      ** Execute assets:environment
      ** Invoke environment (first_time)
      ** Execute environment
      ** Invoke tmp:cache:clear (first_time)
      ** Execute tmp:cache:clear
      ** Execute assets:precompile:primary
      rake aborted!
      LoadError: cannot load such file -- uglifier
        (in /home/cool_tech/cool_tech/app/assets/javascripts/application.js)
      /usr/lib/ruby/gems/2.0.0/gems/activesupport-3.2.14/lib/active_support/dependencies.rb:251:in `require'
      /usr/lib/ruby/gems/2.0.0/gems/activesupport-3.2.14/lib/active_support/dependencies.rb:251:in `block in require'
      /usr/lib/ruby/gems/2.0.0/gems/activesupport-3.2.14/lib/active_support/dependencies.rb:236:in `load_dependency'
      /usr/lib/ruby/gems/2.0.0/gems/activesupport-3.2.14/lib/active_support/dependencies.rb:251:in `require'
      /usr/lib/ruby/gems/2.0.0/gems/actionpack-3.2.14/lib/sprockets/compressors.rb:31:in `registered_js_compressor'
      /usr/lib/ruby/gems/2.0.0/gems/actionpack-3.2.14/lib/sprockets/bootstrap.rb:18:in `block in run'
      /usr/lib/ruby/gems/2.0.0/gems/actionpack-3.2.14/lib/sprockets/compressors.rb:80:in `call'
      /usr/lib/ruby/gems/2.0.0/gems/actionpack-3.2.14/lib/sprockets/compressors.rb:80:in `compressor'
      /usr/lib/ruby/gems/2.0.0/gems/actionpack-3.2.14/lib/sprockets/compressors.rb:74:in `compress'
      /usr/lib/ruby/gems/2.0.0/gems/sprockets-2.2.2/lib/sprockets/processing.rb:265:in `block in js_compressor='
      /usr/lib/ruby/gems/2.0.0/gems/sprockets-2.2.2/lib/sprockets/processor.rb:29:in `call'
      /usr/lib/ruby/gems/2.0.0/gems/sprockets-2.2.2/lib/sprockets/processor.rb:29:in `evaluate'
      /usr/lib/ruby/gems/2.0.0/gems/tilt-1.4.1/lib/tilt/template.rb:103:in `render'
      /usr/lib/ruby/gems/2.0.0/gems/sprockets-2.2.2/lib/sprockets/context.rb:193:in `block in evaluate'
      /usr/lib/ruby/gems/2.0.0/gems/sprockets-2.2.2/lib/sprockets/context.rb:190:in `each'
      /usr/lib/ruby/gems/2.0.0/gems/sprockets-2.2.2/lib/sprockets/context.rb:190:in `evaluate'
      /usr/lib/ruby/gems/2.0.0/gems/sprockets-2.2.2/lib/sprockets/bundled_asset.rb:26:in `initialize'
      /usr/lib/ruby/gems/2.0.0/gems/sprockets-2.2.2/lib/sprockets/base.rb:252:in `new'
      /usr/lib/ruby/gems/2.0.0/gems/sprockets-2.2.2/lib/sprockets/base.rb:252:in `build_asset'
      /usr/lib/ruby/gems/2.0.0/gems/sprockets-2.2.2/lib/sprockets/index.rb:93:in `block in build_asset'
      /usr/lib/ruby/gems/2.0.0/gems/sprockets-2.2.2/lib/sprockets/caching.rb:19:in `cache_asset'
      /usr/lib/ruby/gems/2.0.0/gems/sprockets-2.2.2/lib/sprockets/index.rb:92:in `build_asset'
      /usr/lib/ruby/gems/2.0.0/gems/sprockets-2.2.2/lib/sprockets/base.rb:169:in `find_asset'
      /usr/lib/ruby/gems/2.0.0/gems/sprockets-2.2.2/lib/sprockets/index.rb:60:in `find_asset'
      /usr/lib/ruby/gems/2.0.0/gems/actionpack-3.2.14/lib/sprockets/static_compiler.rb:19:in `block in compile'
      /usr/lib/ruby/gems/2.0.0/gems/sprockets-2.2.2/lib/sprockets/base.rb:219:in `block in each_logical_path'
      /usr/lib/ruby/gems/2.0.0/gems/sprockets-2.2.2/lib/sprockets/base.rb:206:in `block (2 levels) in each_file'
      /usr/lib/ruby/gems/2.0.0/gems/sprockets-2.2.2/lib/sprockets/base.rb:196:in `each'
      /usr/lib/ruby/gems/2.0.0/gems/sprockets-2.2.2/lib/sprockets/base.rb:196:in `each_entry'
      /usr/lib/ruby/gems/2.0.0/gems/sprockets-2.2.2/lib/sprockets/base.rb:204:in `block in each_file'
      /usr/lib/ruby/gems/2.0.0/gems/sprockets-2.2.2/lib/sprockets/base.rb:203:in `each'
      /usr/lib/ruby/gems/2.0.0/gems/sprockets-2.2.2/lib/sprockets/base.rb:203:in `each_file'
      /usr/lib/ruby/gems/2.0.0/gems/sprockets-2.2.2/lib/sprockets/base.rb:217:in `each_logical_path'
      /usr/lib/ruby/gems/2.0.0/gems/actionpack-3.2.14/lib/sprockets/static_compiler.rb:18:in `compile'
      /usr/lib/ruby/gems/2.0.0/gems/actionpack-3.2.14/lib/sprockets/assets.rake:56:in `internal_precompile'
      /usr/lib/ruby/gems/2.0.0/gems/actionpack-3.2.14/lib/sprockets/assets.rake:70:in `block (3 levels) in &lt;top (required)&gt;'
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/lib/rake/task.rb:240:in `call'
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/lib/rake/task.rb:240:in `block in execute'
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/lib/rake/task.rb:235:in `each'
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/lib/rake/task.rb:235:in `execute'
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/lib/rake/task.rb:179:in `block in invoke_with_call_chain'
      /usr/lib/ruby/2.0.0/monitor.rb:211:in `mon_synchronize'
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/lib/rake/task.rb:172:in `invoke_with_call_chain'
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/lib/rake/task.rb:165:in `invoke'
      /usr/lib/ruby/gems/2.0.0/gems/actionpack-3.2.14/lib/sprockets/assets.rake:60:in `block (3 levels) in &lt;top (required)&gt;'
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/lib/rake/task.rb:240:in `call'
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/lib/rake/task.rb:240:in `block in execute'
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/lib/rake/task.rb:235:in `each'
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/lib/rake/task.rb:235:in `execute'
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/lib/rake/task.rb:179:in `block in invoke_with_call_chain'
      /usr/lib/ruby/2.0.0/monitor.rb:211:in `mon_synchronize'
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/lib/rake/task.rb:172:in `invoke_with_call_chain'
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/lib/rake/task.rb:165:in `invoke'
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/lib/rake/application.rb:150:in `invoke_task'
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/lib/rake/application.rb:106:in `block (2 levels) in top_level'
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/lib/rake/application.rb:106:in `each'
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/lib/rake/application.rb:106:in `block in top_level'
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/lib/rake/application.rb:115:in `run_with_threads'
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/lib/rake/application.rb:100:in `top_level'
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/lib/rake/application.rb:78:in `block in run'
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/lib/rake/application.rb:176:in `standard_exception_handling'
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/lib/rake/application.rb:75:in `run'
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/bin/rake:33:in `&lt;top (required)&gt;'
      /usr/bin/rake:23:in `load'
      /usr/bin/rake:23:in `&lt;main&gt;'
      Tasks: TOP =&gt; assets:precompile:primary
      rake aborted!
      Command failed with status (1): [/usr/bin/ruby /usr/bin/rake assets:precomp...]
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/lib/rake/file_utils.rb:54:in `block in create_shell_runner'
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/lib/rake/file_utils.rb:45:in `call'
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/lib/rake/file_utils.rb:45:in `sh'
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/lib/rake/file_utils_ext.rb:37:in `sh'
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/lib/rake/file_utils.rb:82:in `ruby'
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/lib/rake/file_utils_ext.rb:37:in `ruby'
      /usr/lib/ruby/gems/2.0.0/gems/actionpack-3.2.14/lib/sprockets/assets.rake:12:in `ruby_rake_task'
      /usr/lib/ruby/gems/2.0.0/gems/actionpack-3.2.14/lib/sprockets/assets.rake:21:in `invoke_or_reboot_rake_task'
      /usr/lib/ruby/gems/2.0.0/gems/actionpack-3.2.14/lib/sprockets/assets.rake:29:in `block (2 levels) in &lt;top (required)&gt;'
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/lib/rake/task.rb:240:in `call'
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/lib/rake/task.rb:240:in `block in execute'
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/lib/rake/task.rb:235:in `each'
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/lib/rake/task.rb:235:in `execute'
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/lib/rake/task.rb:179:in `block in invoke_with_call_chain'
      /usr/lib/ruby/2.0.0/monitor.rb:211:in `mon_synchronize'
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/lib/rake/task.rb:172:in `invoke_with_call_chain'
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/lib/rake/task.rb:165:in `invoke'
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/lib/rake/application.rb:150:in `invoke_task'
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/lib/rake/application.rb:106:in `block (2 levels) in top_level'
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/lib/rake/application.rb:106:in `each'
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/lib/rake/application.rb:106:in `block in top_level'
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/lib/rake/application.rb:115:in `run_with_threads'
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/lib/rake/application.rb:100:in `top_level'
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/lib/rake/application.rb:78:in `block in run'
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/lib/rake/application.rb:176:in `standard_exception_handling'
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/lib/rake/application.rb:75:in `run'
      /usr/lib/ruby/gems/2.0.0/gems/rake-10.2.2/bin/rake:33:in `&lt;top (required)&gt;'
      /usr/bin/rake:23:in `load'
      /usr/bin/rake:23:in `&lt;main&gt;'
      Tasks: TOP =&gt; assets:precompile
  </code></pre>
  
  <p>Does anybody has any idea what could be wrong? Thanks.</p>