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

lib/active_model/serializer.rb

@@ -124,7 +124,14 @@ module ActiveModel
     end
 
     def attributes(options = {})
-      self.class._attributes.dup.each_with_object({}) do |name, hash|
+      attributes =
+        if options[:fields]
+          self.class._attributes & options[:fields]
+        else
+          self.class._attributes.dup
+        end
+
+      attributes.each_with_object({}) do |name, hash|
         hash[name] = send(name)
       end
     end

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

lib/active_model_serializers.rb

@@ -1,6 +1,7 @@
 require "active_model"
 require "active_model/serializer/version"
 require "active_model/serializer"
+require "active_model/serializer/fieldset"
 
 begin
   require 'action_controller'

test/adapter/json_api/fieldset_test.rb

@@ -0,0 +1,56 @@
+require 'test_helper'
+
+module ActiveModel
+  class Serializer
+    class Adapter
+      class JsonApi
+        class FieldsetTest < Minitest::Test
+          def setup
+            @post = Post.new(title: 'New Post', body: 'Body')
+            @first_comment = Comment.new(id: 1, body: 'ZOMG A COMMENT')
+            @second_comment = Comment.new(id: 2, body: 'ZOMG ANOTHER COMMENT')
+            @post.comments = [@first_comment, @second_comment]
+            @first_comment.post = @post
+            @second_comment.post = @post
+
+            @serializer = PostSerializer.new(@post)
+            @adapter = ActiveModel::Serializer::Adapter::JsonApi.new(@serializer)
+          end
+
+          def teardown
+            @serializer = nil
+            @adapter = nil
+          end
+
+          def test_fieldset_with_fields_array
+            fieldset = ActiveModel::Serializer::Fieldset.new(@serializer, ['title'])
+
+            assert_equal(
+              {:title=>"New Post", :links=>{:comments=>["1", "2"]}}, 
+              @adapter.serializable_hash({fieldset: fieldset})[:posts]
+            )
+          end
+
+          def test_fieldset_with_hash
+            fieldset = ActiveModel::Serializer::Fieldset.new(@serializer, {post: [:body]})
+
+            assert_equal(
+              {:body=>"Body", :links=>{:comments=>["1", "2"]}}, 
+              @adapter.serializable_hash({fieldset: fieldset})[:posts]
+            )
+          end
+
+          def test_fieldset_with_multiple_hashes
+            fieldset = ActiveModel::Serializer::Fieldset.new(@serializer, {post: [:title], comment: [:body]})
+
+            assert_equal(
+              [{:body=>"ZOMG A COMMENT" }, {:body=>"ZOMG ANOTHER COMMENT"}],
+              @adapter.serializable_hash({fieldset: fieldset})[:linked][:comments]
+            )
+          end
+
+        end
+      end
+    end
+  end
+end

test/serializers/attributes_test.rb

@@ -12,7 +12,11 @@ module ActiveModel
         assert_equal([:name, :description],
                      @profile_serializer.class._attributes)
       end
+
+      def test_attributes_with_fields_option
+        assert_equal({name: 'Name 1'},
+                     @profile_serializer.attributes( { fields: [:name] } ) )
+      end
     end
   end
 end
-