WayCost: Add `#cache_key` method6097-cache-route-WayCosts

Provides a uniform Rails cache key for all `WayCost`s using their `id`.
Since the `id` field is option, raise an error if the `WayCost` doesn't
have one.

Refs #6097

1 files changed, 8 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