Aha, this one works

#!/usr/bin/env ruby

require 'httparty'

class Derp
 include HTTParty
  base_uri 'swapi.co/api'

  def initialize(service, page)
    @options = { query: {site: service, page: page} }
  end

  def planets
    self.class.get("/planets", @options)
  end
end

derp = Derp.new("SWAPI", 1)
puts derp.planets

Does not work

require 'active_resource'

class Planet < ActiveResource::Base
  self.site = "http://swapi.co/api/planets/"
end

Planet.find(1)

I think your root should just be http://swapi.co/api/