Updated for Capistrano 3.

deploy.rb

# encoding: UTF-8

# Place in config/deploy.rb

namespace :rails do
  desc "Open the rails console on each of the remote servers"
  task :console do
    on roles(:app) do |host| #does it for each host, bad.
      rails_env = fetch(:stage)
      execute_interactively "ruby #{current_path}/script/rails console #{rails_env}"  
    end
  end

  desc "Open the rails dbconsole on each of the remote servers"
  task :dbconsole do
    on roles(:db) do |host| #does it for each host, bad.
      rails_env = fetch(:stage)
      execute_interactively "ruby #{current_path}/script/rails dbconsole #{rails_env}"  
    end
  end

  def execute_interactively(command)
    user = fetch(:user)
    port = fetch(:port) || 22
    exec "ssh -l #{user} #{host} -p #{port} -t 'cd #{deploy_to}/current && #{command}'"
  end
end

If someone wants to solve how to do this for just the first server of that role, that would be nice. Capistrano 3 is a huge improvement, but it's really quite hard to backwards engineer the rack application if you're not too familiar with rake or ruby. I even went so far as to check out the ObjectSpace in Ruby to figure out what was available to me...

Is this for Rails 4?

@toobulkeh I think that primary(:app).hostname should give the correct hostname.

Change the bad line to:

on roles(:app), :primary => true do

How I'm doing:

cap [staging] rails:console   # connect to the first server
cap [staging] rails:console 1 # connect to the second server

namespace :rails do
  desc 'Start a rails console `cap [staging] rails:console [server_index default: 0]`'
  task :console do
    on roles(:app) do |server|
      server_index = ARGV[2].to_i

      return if server != roles(:app)[server_index]

      puts "Opening a console on: #{host}...."

      cmd = "ssh #{server.user}@#{host} -t 'cd #{fetch(:deploy_to)}/current && RAILS_ENV=#{fetch(:rails_env)} bundle exec rails console'"

      puts cmd

      exec cmd
    end
  end
end

@phstc, +1

i installed rbenv on my server by chef cookbook, i need to add 'source /etc/profile' before cd command. otherwise bundle command is not found.

thanks

In case you have the same setup as me (Rails 4.1.x, Ruby 2.1.x and rbenv) following will do it for you:

namespace :rails do
  desc 'Open the rails console on the primary remote server'
  task :console do
    on roles(:app), primary: true do |host|
      command = "/home/#{host.user}/.rbenv/shims/ruby #{deploy_to}/current/bin/rails console #{fetch(:stage)}"
      exec "ssh -l #{host.user} #{host.hostname} -p #{host.port} -t 'cd #{deploy_to}/current && #{command}'"
    end
  end
end

Here's what I found worked with Rails 4.1, Ruby 2.1, and RVM with the capistrano-rvm extension

namespace :rails do
  desc "Open the rails console on each of the remote servers"
  task :console => 'rvm:hook' do
    on roles(:app), :primary => true do |host|
      rails_env = fetch(:stage)
      execute_interactively host, "console #{rails_env}"
    end
  end

  desc "Open the rails dbconsole on each of the remote servers"
  task :dbconsole => 'rvm:hook' do
    on roles(:app), :primary => true do |host|
      rails_env = fetch(:stage)
      execute_interactively host, "dbconsole #{rails_env}"
    end
  end

  def execute_interactively(host, command)
    command = "cd #{fetch(:deploy_to)}/current && #{SSHKit.config.command_map[:bundle]} exec rails #{command}"
    puts command if fetch(:log_level) == :debug
    exec "ssh -l #{host.user} #{host.hostname} -p #{host.port || 22} -t '#{command}'"
  end
end

Gist: https://gist.github.com/chrisbloom7/5de890debc104390d6d4

What about running it just on the first available host.

namespace :rails do
  desc "Open the rails console on each of the remote servers"
  task :console do
    on roles(:app).first do |host| #does it for the first host.
      rails_env = fetch(:stage)
      execute_interactively "ruby #{current_path}/script/rails console #{rails_env}"  
    end
  end

  desc "Open the rails dbconsole on each of the remote servers"
  task :dbconsole do
    on roles(:db).first do |host| #does it for the first host.
      rails_env = fetch(:stage)
      execute_interactively "ruby #{current_path}/script/rails dbconsole #{rails_env}"  
    end
  end

  def execute_interactively(command)
    user = fetch(:user)
    port = fetch(:port) || 22
    exec "ssh -l #{user} #{host} -p #{port} -t 'cd #{deploy_to}/current && #{command}'"
  end
end

For rbenv:

namespace :rails do
  desc "Open the rails console"
  task :console do
    on roles(:app) do
      rails_env = fetch(:rails_env, 'production')
      execute_interactively "$HOME/.rbenv/bin/rbenv exec bundle exec rails console #{rails_env}"
    end
  end

  desc "Open the rails dbconsole"
  task :dbconsole do
    on roles(:app) do
      rails_env = fetch(:rails_env, 'production')
      execute_interactively "$HOME/.rbenv/bin/rbenv exec bundle exec rails dbconsole #{rails_env}"
    end
  end

  def execute_interactively(command)
    user = fetch(:user)
    port = fetch(:port) || 22
    exec "ssh -l #{user} #{host} -p #{port} -t 'cd #{deploy_to}/current && #{command}'"
  end
end

In case anyone still have issues with rbenv and @DmytroLukianov solution, I tuned it a bit and it worked for me:

namespace :rails do
  desc 'Start a rails console'
  # Usage: cap production rails:console
  task :console do
    on roles(:app) do
      exec "ssh #{host.user}@#{host.hostname} -p #{host.port || 22} -t 'cd #{current_path} && $HOME/.rbenv/bin/rbenv exec bundle exec rails console'"
    end
  end
end