Lazify calculating caller file digest until used

CHANGELOG.md

@@ -8,6 +8,7 @@ Breaking changes:
 - [#1574](https://github.com/rails-api/active_model_serializers/pull/1574) Default key case for the JsonApi adapter changed to dashed. (@remear)
 
 Features:
+- [#1687](https://github.com/rails-api/active_model_serializers/pull/1687) Only calculate `_cache_digest` (in `cache_key`) when `skip_digest` is false. (@bf4)
 - [#1647](https://github.com/rails-api/active_model_serializers/pull/1647) Restrict usage of `serializable_hash` options
   to the ActiveModel::Serialization and ActiveModel::Serializers::JSON interface. (@bf4)
 - [#1645](https://github.com/rails-api/active_model_serializers/pull/1645) Transform keys referenced in values. (@remear)

lib/active_model/serializer/caching.rb

@@ -18,8 +18,9 @@ module ActiveModel
           #    force
           #    race_condition_ttl
           #  Passed to ::_cache as
-          #    serializer._cache.fetch(cache_key, @klass._cache_options)
-          serializer.class_attribute :_cache_digest # @api private : Generated
+          #    serializer.cache_store.fetch(cache_key, @klass._cache_options)
+          #  Passed as second argument to serializer.cache_store.fetch(cache_key, self.class._cache_options)
+          serializer.class_attribute :_cache_digest_file_path # @api private : Derived at inheritance
         end
       end
 
@@ -42,7 +43,12 @@ module ActiveModel
         def inherited(base)
           super
           caller_line = caller[1]
-          base._cache_digest = digest_caller_file(caller_line)
+          base._cache_digest_file_path = caller_line
+        end
+
+        def _cache_digest
+          return @_cache_digest if defined?(@_cache_digest)
+          @_cache_digest = digest_caller_file(_cache_digest_file_path)
         end
 
         # Hashes contents of file for +_cache_digest+