Includes links inside of linked resources

According to
http://jsonapi.org/format/#document-structure-resource-objects,

> Resource objects have the same internal structure, regardless of
> whether they represent primary or linked resources.

And then
http://jsonapi.org/format/#document-structure-resource-object-attributes,

> There are four reserved keys in resource objects:
>
> "id"
> "type"
> "href"
> "links"

This commits includes `links` inside of linked resources.

lib/active_model/serializer/adapter/json_api.rb

@@ -19,69 +19,57 @@ module ActiveModel
           else
             @hash[@root] = attributes_for_serializer(serializer, @options)
 
-            serializer.each_association do |name, association, opts|
-              @hash[@root][:links] ||= {}
-
-              if association.respond_to?(:each)
-                add_links(name, association, opts)
-              else
-                add_link(name, association, opts)
-              end
-            end
+            add_resource_links(@hash[@root], serializer)
           end
 
           @hash
         end
 
-        def add_links(name, serializers, options)
-          if serializers.first
-            type = serializers.first.object.class.to_s.underscore.pluralize
-          end
+        private
+
+        def add_links(resource, name, serializers)
+          type = serialized_object_type(serializers)
+          resource[:links] ||= {}
+
           if name.to_s == type || !type
-            @hash[@root][:links][name] ||= []
-            @hash[@root][:links][name] += serializers.map{|serializer| serializer.id.to_s }
+            resource[:links][name] ||= []
+            resource[:links][name] += serializers.map{|serializer| serializer.id.to_s }
           else
-            @hash[@root][:links][name] ||= {}
-            @hash[@root][:links][name][:type] = type
-            @hash[@root][:links][name][:ids] ||= []
-            @hash[@root][:links][name][:ids] += serializers.map{|serializer| serializer.id.to_s }
-          end
-
-          unless serializers.none? || @options[:embed] == :ids
-            serializers.each do |serializer|
-              add_linked(name, serializer)
-            end
+            resource[:links][name] ||= {}
+            resource[:links][name][:type] = type
+            resource[:links][name][:ids] ||= []
+            resource[:links][name][:ids] += serializers.map{|serializer| serializer.id.to_s }
           end
         end
 
-        def add_link(name, serializer, options)
+        def add_link(resource, name, serializer)
+          resource[:links] ||= {}
+          resource[:links][name] = nil
+
           if serializer
-            type = serializer.object.class.to_s.underscore
+            type = serialized_object_type(serializer)
             if name.to_s == type || !type
-              @hash[@root][:links][name] = serializer.id.to_s
+              resource[:links][name] = serializer.id.to_s
             else
-              @hash[@root][:links][name] ||= {}
-              @hash[@root][:links][name][:type] = type
-              @hash[@root][:links][name][:id] = serializer.id.to_s
+              resource[:links][name] ||= {}
+              resource[:links][name][:type] = type
+              resource[:links][name][:id] = serializer.id.to_s
             end
-
-            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
+        def add_linked(resource_name, serializer, parent = nil)
+          resource_path = [parent, resource_name].compact.join('.')
+
+          if include_assoc?(resource_path)
+            plural_name = resource_name.to_s.pluralize.to_sym
             attrs = [attributes_for_serializer(serializer, @options)].flatten
             @top[:linked] ||= {}
             @top[:linked][plural_name] ||= []
 
             attrs.each do |attrs|
+              add_resource_links(attrs, serializer, add_linked: false)
+
               @top[:linked][plural_name].push(attrs) unless @top[:linked][plural_name].include?(attrs)
             end
           end
@@ -91,8 +79,6 @@ module ActiveModel
           end if include_nested_assoc? resource_path
         end
 
-        private
-
         def attributes_for_serializer(serializer, options)
           if serializer.respond_to?(:each)
             result = []
@@ -124,6 +110,36 @@ module ActiveModel
             s.match(/^#{assoc.gsub('.', '\.')}/)
           end
         end
+
+        def serialized_object_type(serializer)
+          return false unless Array(serializer).first
+          type_name = Array(serializer).first.object.class.to_s.underscore
+          if serializer.respond_to?(:first)
+            type_name.pluralize
+          else
+            type_name
+          end
+        end
+
+        def add_resource_links(attrs, serializer, options = {})
+          options[:add_linked] = options.fetch(:add_linked, true)
+
+          Array(serializer).first.each_association do |name, association, opts|
+            attrs[:links] ||= {}
+
+            if association.respond_to?(:each)
+              add_links(attrs, name, association)
+            else
+              add_link(attrs, name, association)
+            end
+
+            if @options[:embed] != :ids && options[:add_linked]
+              Array(association).each do |association|
+                add_linked(name, association)
+              end
+            end
+          end
+        end
       end
     end
   end