Backport caching of the constant lookup

lib/active_model/serializable/utils.rb

@@ -4,13 +4,15 @@ module ActiveModel
       extend self
 
       def _const_get(const)
-        begin
-          method = RUBY_VERSION >= '2.0' ? :const_get : :qualified_const_get
-          Object.send method, const
-        rescue NameError
-          const.safe_constantize
+        Serializer.serializers_cache.fetch_or_store(const) do
+          begin
+            method = RUBY_VERSION >= '2.0' ? :const_get : :qualified_const_get
+            Object.send method, const
+          rescue NameError
+            const.safe_constantize
+          end
         end
       end
     end
   end
-end
+end

lib/active_model/serializer.rb

@@ -4,6 +4,7 @@ require 'active_model/serializer/association'
 require 'active_model/serializer/config'
 
 require 'thread'
+require 'concurrent/map'
 
 module ActiveModel
   class Serializer
@@ -100,6 +101,10 @@ end
         associate(Association::HasMany, *attrs)
       end
 
+      def serializers_cache
+        @serializers_cache ||= Concurrent::Map.new
+      end
+
       private
 
       def strip_attribute(attr)