Include 'linked' member for json-api collections

The options passed to the render are partitioned into adapter options
and serializer options. 'include' and 'root' are sent to the adapter,
not sure what options would go directly to serializer, but leaving this
in until I understand that better.

lib/active_model/serializer/adapter/json_api.rb

@@ -5,16 +5,19 @@ module ActiveModel
         def initialize(serializer, options = {})
           super
           serializer.root = true
+          @hash = {}
+          @top = @options.fetch(:top) { @hash }
         end
 
         def serializable_hash(options = {})
-          @root = (options[:root] || serializer.json_key.to_s.pluralize).to_sym
-          @hash = {}
+          @root = (@options[:root] || serializer.json_key.to_s.pluralize).to_sym
 
           if serializer.respond_to?(:each)
-            @hash[@root] = serializer.map{|s| self.class.new(s).serializable_hash[@root] }
+            @hash[@root] = serializer.map do |s|
+              self.class.new(s, @options.merge(top: @top)).serializable_hash[@root]
+            end
           else
-            @hash[@root] = attributes_for_serializer(serializer, {})
+            @hash[@root] = attributes_for_serializer(serializer, @options)
 
             serializer.each_association do |name, association, opts|
               @hash[@root][:links] ||= {}
@@ -44,10 +47,10 @@ module ActiveModel
             @hash[@root][:links][name][:ids] += serializers.map{|serializer| serializer.id.to_s }
           end
 
-          unless options[:embed] == :ids || serializers.count == 0
-            @hash[:linked] ||= {}
-            @hash[:linked][name] ||= []
-            @hash[:linked][name] += serializers.map { |item| attributes_for_serializer(item, options) }
+          unless serializers.none? || @options[:embed] == :ids
+            serializers.each do |serializer|
+              add_linked(name, serializer)
+            end
           end
         end
 
@@ -62,17 +65,31 @@ module ActiveModel
               @hash[@root][:links][name][:id] = serializer.id.to_s
             end
 
-            unless options[:embed] == :ids
-              plural_name = name.to_s.pluralize.to_sym
-              @hash[:linked] ||= {}
-              @hash[:linked][plural_name] ||= []
-              @hash[:linked][plural_name].push attributes_for_serializer(serializer, options)
+            unless @options[:embed] == :ids
+              add_linked(name, serializer)
             end
           else
             @hash[@root][:links][name] = nil
           end
         end
 
+        def add_linked(resource, serializer, parent = nil)
+          resource_path = [parent, resource].compact.join('.')
+          if include_assoc? resource_path
+            plural_name = resource.to_s.pluralize.to_sym
+            attrs = attributes_for_serializer(serializer, @options)
+            @top[:linked] ||= {}
+            @top[:linked][plural_name] ||= []
+            @top[:linked][plural_name].push attrs unless @top[:linked][plural_name].include? attrs
+          end
+
+          unless serializer.respond_to?(:each)
+            serializer.each_association do |name, association, opts|
+              add_linked(name, association, resource) if association
+            end
+          end
+        end
+
         private
 
         def attributes_for_serializer(serializer, options)
@@ -80,6 +97,10 @@ module ActiveModel
           attributes[:id] = attributes[:id].to_s if attributes[:id]
           attributes
         end
+
+        def include_assoc? assoc
+          @options[:include] && @options[:include].split(',').include?(assoc.to_s)
+        end
       end
     end
   end