* Configure adapter using ActiveModel::Serializer.config.adapter

* Get adapter instance using ActiveModel::Serializer::Adapter.adapter_for(serializer)

lib/active_model/serializer/adapter.rb

@@ -16,6 +16,24 @@ module ActiveModel
       def to_json(options = {})
         raise NotImplementedError, 'This is abstract method. Should be implemented at concrete adapter.'
       end
+
+      def self.adapter_for(serializer)
+        adapter_class = case serializer.config.adapter
+                        when Symbol
+                          class_name = "ActiveModel::Serializer::Adapter::#{serializer.config.adapter.to_s.classify}Adapter"
+                          if Object.const_defined?(class_name)
+                            Object.const_get(class_name)
+                          end
+                        when Class
+                          serializer.config.adapter
+                        end
+        unless adapter_class
+          valid_adapters = self.constants.map { |klass| ":#{klass.to_s.sub('Adapter', '').downcase}" }
+          raise ArgumentError, "Unknown adapter: #{serializer.config.adapter}. Valid adapters are: #{valid_adapters}"
+        end
+
+        adapter_class.new(serializer)
+      end
     end
   end
 end