routes.rb

class QueryParameterConstraint
  def initialize(*keys)
    @keys = keys
  end

  def matches?(request)
    result = false
    @keys.each do |key|
      result = request.query_parameters.has_key?(key)
      break unless result
    end
    result
  end
end

MyApp::Application.routes.draw do
  resources :posts do
    get '/', to: :search, as: :search, on: :collection,
      constraints: QueryParameterConstraint.new(:q)
  end
end