Running the command during a capistrano deploy:
bundle exec rake RAILS_ENV=staging RAILS_GROUPS=assets assets:precompile
returns
rake aborted!
couldn't find file 'handlebars'
(in */app/assets/javascripts/application.js:22)
*/ruby/1.9.1/gems/sprockets-2.2.2/lib/sprockets/context.rb:102:in `resolve'
*/ruby/1.9.1/gems/sprockets-2.2.2/lib/sprockets/context.rb:142:in `require_asset'
*************
Tasks: TOP => assets:precompile:all
Gem ember-rails is in a global scope in my Gemfile and I am using handlebars-source (1.0.12), ember-rails (0.12.0), ember-source (1.0.0.rc6.2) in Rails 3.2.12 and ruby 1.9.3. It's probably worth noting that this works in production, but not in staging deploys.
Also I have the following in my staging.rb, which appears to be the common issue.
config.ember.variant = :production