Backport caching of the constant lookup

active_model_serializers.gemspec

@@ -21,5 +21,6 @@ Gem::Specification.new do |gem|
   gem.required_ruby_version = ">= 1.9.3"
 
   gem.add_dependency "activemodel", ">= 3.2"
+  gem.add_dependency "concurrent-ruby", "~> 1.0"
   gem.add_development_dependency "rails", ">= 3.2"
 end

lib/active_model/serializable.rb

@@ -33,7 +33,11 @@ def serializable_data
     end
 
     def namespace
-      get_namespace && Utils._const_get(get_namespace)
+      if module_name = get_namespace
+        Serializer.serializers_cache.fetch_or_store(module_name) do
+          Utils._const_get(module_name)
+        end
+      end
     end
 
     def embedded_in_root_associations

lib/active_model/serializer.rb

@@ -4,6 +4,7 @@
 require 'active_model/serializer/config'
 
 require 'thread'
+require 'concurrent/map'
 
 module ActiveModel
   class Serializer
@@ -65,7 +66,10 @@ def serializer_for(resource, options = {})
             ArraySerializer
           end
         else
-          _const_get build_serializer_class(resource, options)
+          klass_name = build_serializer_class(resource, options)
+          Serializer.serializers_cache.fetch_or_store(klass_name) do
+            _const_get(klass_name)
+          end
         end
       end
 
@@ -100,6 +104,10 @@ def has_many(*attrs)
         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 @@
       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