2 files changed, 30 insertions, 0 deletions
diff --git a/lib/way_cost.rb b/lib/way_cost.rb
index 9f860308c..53e69ddb3 100644
--- a/lib/way_cost.rb
+++ b/lib/way_cost.rb
@@ -24,4 +24,12 @@ class WayCost
       @time == other.time &&
       @id == other.id
   end
+
+  def cache_key
+    raise NullIDError if !id
+
+    "way_cost/#{id}"
+  end
+
+  class NullIDError < RuntimeError; end
 end
diff --git a/spec/lib/way_cost_spec.rb b/spec/lib/way_cost_spec.rb
new file mode 100644
index 000000000..1e85c68ab
--- /dev/null
+++ b/spec/lib/way_cost_spec.rb
@@ -0,0 +1,22 @@
+RSpec.describe WayCost do
+  describe "#cache_key" do
+    it "returns a cache string including the ID of the WayCost" do
+      way_cost = WayCost.new(
+        departure: Geokit::LatLng.new(47.91231, 1.87606),
+        arrival: Geokit::LatLng.new(52.50867, 13.42879),
+        id: '2-3'
+      )
+
+      expect(way_cost.cache_key).to eq('way_cost/2-3')
+    end
+
+    it "raises an error if the WayCost doesn't have an ID" do
+      way_cost = WayCost.new(
+        departure: Geokit::LatLng.new(47.91231, 1.87606),
+        arrival: Geokit::LatLng.new(52.50867, 13.42879)
+      )
+
+      expect { way_cost.cache_key }.to raise_error(WayCost::NullIDError)
+    end
+  end
+end