Restore cache_read_multi, though benchmark does not show improvement

Benchmark results:
{
  "commit_hash": "33ae947",
  "version": "0.10.0.rc5",
  "rails_version": "4.2.6",
  "benchmark_run[environment]": "2.2.2p95",
  "runs": [
    {
      "benchmark_type[category]": "caching on: caching serializers: gc off",
      "benchmark_run[result][iterations_per_second]": 677.638,
      "benchmark_run[result][total_allocated_objects_per_iteration]": 1795
    },
    {
      "benchmark_type[category]": "caching off: caching serializers: gc off",
      "benchmark_run[result][iterations_per_second]": 647.854,
      "benchmark_run[result][total_allocated_objects_per_iteration]": 1795
    },
    {
      "benchmark_type[category]": "caching on: non-caching serializers: gc off",
      "benchmark_run[result][iterations_per_second]": 936.023,
      "benchmark_run[result][total_allocated_objects_per_iteration]": 1252
    },
    {
      "benchmark_type[category]": "caching off: non-caching serializers: gc off",
      "benchmark_run[result][iterations_per_second]": 882.823,
      "benchmark_run[result][total_allocated_objects_per_iteration]": 1252
    }
}

Author: bf4

.rubocop.yml

@@ -30,6 +30,9 @@ Metrics/ClassLength:
   Exclude:
     - test/**/*.rb
 
+Metrics/ModuleLength:
+  Max: 103 # TODO: Lower to 100
+
 Metrics/CyclomaticComplexity:
   Max: 7 # TODO: Lower to 6
 

lib/active_model/serializer/caching.rb

@@ -17,8 +17,7 @@ module Caching
           #    compress
           #    force
           #    race_condition_ttl
-          #  Passed to ::_cache as
-          #    serializer._cache.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 # @api private : Generated
         end
       end
@@ -208,8 +207,12 @@ def cached_attributes(fields, adapter_instance)
 
       def cache_check(adapter_instance)
         if self.class.cache_enabled?
-          self.class.cache_store.fetch(cache_key(adapter_instance), self.class._cache_options) do
-            yield
+          cached_attributes = instance_options[:cached_attributes] || {}
+          key = cache_key(adapter_instance)
+          cached_attributes.fetch(key) do
+            self.class.cache_store.fetch(key, self.class._cache_options) do
+              yield
+            end
           end
         elsif self.class.fragment_cache_enabled?
           fetch_fragment_cache(adapter_instance)

lib/active_model_serializers/adapter/attributes.rb

@@ -19,6 +19,7 @@ def serializable_hash(options = nil)
       private
 
       def serializable_hash_for_collection(options)
+        instance_options[:cached_attributes] ||= ActiveModel::Serializer.cache_read_multi(serializer, self, @include_tree)
         serializer.map { |s| Attributes.new(s, instance_options).serializable_hash(options) }
       end