Backport caching of the constant lookup

lib/active_model/serializable/utils.rb

@@ -4,6 +4,7 @@ module ActiveModel
       extend self
 
       def _const_get(const)
+        Serializer.serializers_cache.fetch_or_store(const) do
           begin
             method = RUBY_VERSION >= '2.0' ? :const_get : :qualified_const_get
             Object.send method, const
@@ -13,4 +14,5 @@ module ActiveModel
         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)

lib/active_model_serializers.rb

@@ -14,6 +14,9 @@ begin
       ActionController::Base.send(:include, ::ActionController::Serialization)
       ActionController::TestCase.send(:include, ::ActionController::SerializationAssertions)
     end
+    ActionDispatch::Reloader.to_prepare do
+      ActiveModel::Serializer.serializers_cache.clear
+    end
   end
 rescue LoadError
   # rails not installed, continuing