How do I use Rack::Proxy within Rails to proxy requests to a specific path to another app

FWIW, I also just tackled this problem. Some may find the full code helpful, as I needed more than you posted:

# lib/proxy_to_other.rb
class ProxyToOther < Rack::Proxy
  def initialize(app)
    @app = app
  end

  def call(env)
    original_host = env["HTTP_HOST"]
    rewrite_env(env)
    if env["HTTP_HOST"] != original_host
      perform_request(env)
    else
      # just regular
      @app.call(env)
    end
  end

  def rewrite_env(env)
    request = Rack::Request.new(env)
    if request.path =~ %r{^/prefix|^/other_prefix}
      # do nothing
    else
      env["HTTP_HOST"] = "localhost:3000"
    end
    env
  end
end

Also:

# config/application.rb
# ...snip ...
module MyApplication
  class Application < Rails::Application
    # Custom Rack middlewares
    config.middleware.use "ProxyToOther" if ["development", "test"].include? Rails.env
#...snip....

This assumes your app you want to proxy some requests to is running on port 3001. I daresay the app you're hitting can be run on any port. This also assumes you only want to do the proxying in development and test environments, because you'll have a 'real' solution in production & staging (eg, nginx or a loadbalancer doing the proper thing).

This is a slight change to steve's solution that uses a little less internal understanding of Rack::Proxy:

require 'rack/proxy'

class MyProxy < Rack::Proxy
  def initialize(app)
    @app = app
  end

  def call(env)
    # call super if we want to proxy, otherwise just handle regularly via call
    (proxy?(env) && super) || @app.call(env)
  end

  def proxy?(env)
    # do not alter env here, but return true if you want to proxy for this request.
    return true
  end

  def rewrite_env(env)
    # change the env here
    env["HTTP_HOST"] = "some.other.host"
    env
  end
end

Figured it out.

lib/proxy.rb

require 'rack-proxy'
class Proxy < Rack::Proxy
    def initialize(app)
        @app = app
    end

    def rewrite_env(env)
        # do magic in here
    end
 end

config/application.rb

config.middleware.use "Proxy"