implement sparse fieldsets http://jsonapi.org/format/#fetching-sparse-fieldsets

lib/active_model/serializer/adapter.rb

@@ -17,6 +17,7 @@ module ActiveModel
       end
 
       def to_json(options = {})
+        options[:fieldset] = ActiveModel::Serializer::Fieldset.new(serializer, options[:fields])
         serializable_hash(options).to_json
       end
     end

lib/active_model/serializer/adapter/json_api.rb

@@ -10,9 +10,12 @@ module ActiveModel
         def serializable_hash(options = {})
           @root = (options[:root] || serializer.json_key).to_s.pluralize.to_sym
           @hash = {}
+          @fieldset = options[:fieldset]
 
           if serializer.respond_to?(:each)
-            @hash[@root] = serializer.map{|s| self.class.new(s).serializable_hash[@root] }
+            opt = @fieldset ? {fieldset: @fieldset} : {}
+        
+            @hash[@root] = serializer.map{|s| self.class.new(s).serializable_hash(opt)[@root] }
           else
             @hash[@root] = attributes_for_serializer(serializer, {})
 
@@ -57,6 +60,11 @@ module ActiveModel
         private
 
         def attributes_for_serializer(serializer, options)
+
+          if fields = @fieldset && @fieldset.fields_for(serializer)
+            options[:fields] = fields
+          end
+
           attributes = serializer.attributes(options)
           attributes[:id] = attributes[:id].to_s if attributes[:id]
           attributes

lib/active_model/serializer/fieldset.rb

@@ -0,0 +1,33 @@
+module ActiveModel
+  class Serializer
+    class Fieldset
+
+      attr_accessor :fields, :root
+
+      def initialize(serializer, fields = {})
+        @root   = serializer.json_key
+        @fields = parse(fields)
+      end
+
+      def fields_for(serializer)
+        key = serializer.json_key || serializer.class.root_name
+        fields[key]
+      end
+
+    private
+
+      def parse(fields)
+        if fields.is_a?(Hash)
+          fields.inject({}) { |h,(k,v)| h[k.to_s] = v.map(&:to_sym); h}
+        elsif fields.is_a?(Array)
+          hash = {}
+          hash[root.to_s] = fields.map(&:to_sym)
+          hash
+        else
+          {}
+        end
+      end
+
+    end
+  end
+end