rspec

controller

  def new
    begin
      @slug = params[:slug_text]
      @place = Slug.where(name: @slug).first.sluggable
      @klass = @place.class
    rescue Exception => e
      redirect_to :root
    end
  end

spec

describe ModerationController do
  describe "#new" do

    it "sends the query to the correct class" do
      place=stub(:place)
      Slug.should_receive(:where).with(name: "Paris").and_return(place)
      get :new, slug_text: "Paris"
    end

  end