A Rake task that generates Heroku's .gems file from the existing Gem configuration

heroku.rake

namespace :heroku do
  desc "Generates the .gems dependencies from the config.gem directives of the current environment"
  task :gems => :environment do
    File.open(".gems", "w") do |io|
      # If we have vendored Rails, no need to specify a dependency
      io.printf "%-30s --version %-12s\n", "rails", Rails.version.to_s.inspect unless Rails.vendor_rails?
      
      Rails.configuration.gems.each do |gem|
        next if gem.vendor_gem? # Don't install gems that we've vendored
        
        name            = gem.name
        versions        = gem.requirement ? gem.requirement.as_list : []
        current_version = Gem.loaded_specs[name].version
        source          = gem.source
        
        io.printf "%-30s", name
        if versions.empty?
          io.printf " --version %-12s", current_version.to_s.inspect
        else
          versions.each do |version|
            io.printf " --version %-12s", version.to_s.inspect
          end
        end
        io.printf " --source \"%s\"", source if source
        io.printf "\n"
      end
    end
    
    sh "git add .gems"
    sh "git commit -m \"Updated .gems dependencies for Heroku, in the #{Rails.env} environment\" .gems"
  end
end