gistfile1.rb

  describe "named scope" do

    describe "#similar_to" do

      let(:category) { Factory.create(:category) }

      let(:product) { 
        Factory.create(
          :product, :categories => [category]
        )
      }

      subject { described_class.similar_to(product) }

      let(:similar_product) {
        Factory.create(:product, :categories => [category])
      }

      let(:not_similar_product) {
        Factory.create(:product, :categories => [Factory.create(:category)])
      }

      it { should discover(similar_product.group) }
      it { should_not discover(not_similar_product.group) }
    end

Ultimate rspec matcher to test ActiveRecord named scopes