Round petition cache key

When the site it busy the cache key for the petition is being constantly
expired due to rapid changes to updated_at. By rounding the timestamp
down to the nearest 5 seconds it means that the cache won't be invalidated
at a rate more than 12 times per minute.

app/models/petition.rb

@@ -507,6 +507,24 @@ def set_petition_on_creator_signature
     creator_signature.update_attribute(:petition_id, id)
   end
 
+  def cache_key(*timestamp_names)
+    case
+    when new_record?
+      "petitions/new"
+    when timestamp_names.any?
+      timestamp = max_updated_column_timestamp(timestamp_names)
+      timestamp = timestamp.change(sec: (timestamp.sec.div(5) * 5))
+      timestamp = timestamp.utc.to_s(cache_timestamp_format)
+      "petitions/#{id}-#{timestamp}"
+    when timestamp = max_updated_column_timestamp
+      timestamp = timestamp.change(sec: (timestamp.sec.div(5) * 5))
+      timestamp = timestamp.utc.to_s(cache_timestamp_format)
+      "petitions/#{id}-#{timestamp}"
+    else
+      "petitions/#{id}"
+    end
+  end
+
   def update_last_petition_created_at
     Site.touch(:last_petition_created_at)
   end

spec/models/petition_spec.rb

@@ -1727,5 +1727,22 @@
         expect(petition.signatures_to_email_for('government_response')).to match_array [creator_signature, other_signature]
       end
     end
+
+    describe "#cache_key" do
+      let(:petition) { FactoryGirl.create(:petition, last_signed_at: "2016-06-28 00:00:17 UTC", open_at: "2016-06-28 00:00:07 UTC") }
+      let(:now) { "2016-06-29 00:00:07 UTC".in_time_zone }
+
+      around do |example|
+        travel_to(now) { example.run }
+      end
+
+      it "rounds down to the nearest 5 seconds" do
+        expect(petition.cache_key).to eq("petitions/#{petition.id}-20160629000005000000000")
+      end
+
+      it "can use other columns" do
+        expect(petition.cache_key(:open_at, :last_signed_at)).to eq("petitions/#{petition.id}-20160628000015000000000")
+      end
+    end
   end
 end