Adding Fragment Cache to AMS

It's an upgrade based on the new Cache implementation #693.
It allows to use the Rails conventions to cache
specific attributes or associations.
It's based on the Cache Composition implementation.

lib/active_model/serializer/adapter/fragment_cache.rb

@@ -0,0 +1,78 @@
+module ActiveModel
+  class Serializer
+    class Adapter
+      class FragmentCache
+
+        attr_reader :serializer
+
+        def initialize(adapter, serializer, options, root)
+          @root       = root
+          @options    = options
+          @adapter    = adapter
+          @serializer = serializer
+        end
+
+        def fetch
+          klass = serializer.class
+          # It will split the serializer into two, one that will be cached and other wont
+          serializers = fragment_serializer(serializer.object.class.name, klass)
+
+          # Instanciate both serializers
+          cached_serializer     = serializers[:cached].constantize.new(serializer.object)
+          non_cached_serializer = serializers[:non_cached].constantize.new(serializer.object)
+
+          cached_adapter     = @adapter.class.new(cached_serializer, @options)
+          non_cached_adapter = @adapter.class.new(non_cached_serializer, @options)
+
+          # Get serializable hash from both
+          cached_hash     = cached_adapter.serializable_hash
+          non_cached_hash = non_cached_adapter.serializable_hash
+
+          # Merge both results
+          @adapter.fragment_cache(cached_hash, non_cached_hash)
+        end
+
+        private
+
+        def cached_attributes(klass, serializers)
+          cached_attributes     = (klass._cache_only) ? klass._cache_only : serializer.attributes.keys.delete_if {|attr| klass._cache_except.include?(attr) }
+          non_cached_attributes = serializer.attributes.keys.delete_if {|attr| cached_attributes.include?(attr) }
+
+          cached_attributes.each do |attribute|
+            options = serializer.class._attributes_keys[attribute]
+            options ||= {}
+            # Add cached attributes to cached Serializer
+            serializers[:cached].constantize.attribute(attribute, options)
+          end
+
+          non_cached_attributes.each do |attribute|
+            options = serializer.class._attributes_keys[attribute]
+            options ||= {}
+            # Add non-cached attributes to non-cached Serializer
+            serializers[:non_cached].constantize.attribute(attribute, options)
+          end
+        end
+
+        def fragment_serializer(name, klass)
+          cached     = "#{name.capitalize}CachedSerializer"
+          non_cached = "#{name.capitalize}NonCachedSerializer"
+
+          Object.const_set cached, Class.new(ActiveModel::Serializer) unless Object.const_defined?(cached)
+          Object.const_set non_cached, Class.new(ActiveModel::Serializer) unless Object.const_defined?(non_cached)
+
+          klass._cache_options       ||= {}
+          klass._cache_options[:key] = klass._cache_key if klass._cache_key
+
+          cached.constantize.cache(klass._cache_options)
+
+          cached.constantize.fragmented(serializer)
+          non_cached.constantize.fragmented(serializer)
+
+          serializers = {cached: cached, non_cached: non_cached}
+          cached_attributes(klass, serializers)
+          serializers
+        end
+      end
+    end
+  end
+end