Added serializer file digest to the cache_key

Fixes #901

lib/active_model/serializer.rb

@@ -19,6 +19,7 @@ module ActiveModel
       attr_accessor :_cache_only
       attr_accessor :_cache_except
       attr_accessor :_cache_options
+      attr_accessor :_cache_digest
     end
 
     def self.inherited(base)
@@ -26,6 +27,8 @@ module ActiveModel
       base._attributes_keys = {}
       base._associations = {}
       base._urls = []
+      serializer_file = File.open(caller.first[/^[^:]+/])
+      base._cache_digest = Digest::MD5.hexdigest(serializer_file.read)
     end
 
     def self.attributes(*attrs)

lib/active_model/serializer/adapter.rb

@@ -63,6 +63,13 @@ module ActiveModel
       end
 
       def cache_key
+        parts = []
+        parts << object_cache_key
+        parts << @klass._cache_digest unless @klass._cache_options && @klass._cache_options[:skip_digest]
+        parts.join("/")
+      end
+
+      def object_cache_key
         (@klass._cache_key) ? "#{@klass._cache_key}/#{@cached_serializer.object.id}-#{@cached_serializer.object.updated_at}" : @cached_serializer.object.cache_key
       end