How do I run a rake task from Capistrano?

Capistrano 3 Generic Version (run any rake task)

Building a generic version of Mirek Rusin's answer:

desc 'Invoke a rake command on the remote server'
task :invoke, [:command] => 'deploy:set_rails_env' do |task, args|
  on primary(:app) do
    within current_path do
      with :rails_env => fetch(:rails_env) do
        rake args[:command]
      end
    end
  end
end

Example usage: cap staging "invoke[db:migrate]"

Note that deploy:set_rails_env requires comes from the capistrano-rails gem

A little bit more explicit, in your \config\deploy.rb, add outside any task or namespace:

namespace :rake do  
  desc "Run a task on a remote server."  
  # run like: cap staging rake:invoke task=a_certain_task  
  task :invoke do  
    run("cd #{deploy_to}/current; /usr/bin/env rake #{ENV['task']} RAILS_ENV=#{rails_env}")  
  end  
end

Then, from /rails_root/, you can run:

cap staging rake:invoke task=rebuild_table_abc