Add DSL for urls

lib/active_model/serializer.rb

@@ -9,11 +9,13 @@ module ActiveModel
     class << self
       attr_accessor :_attributes
       attr_accessor :_associations
+      attr_accessor :_urls
     end
 
     def self.inherited(base)
       base._attributes = []
       base._associations = {}
+      base._urls = []
     end
 
     def self.attributes(*attrs)
@@ -62,6 +64,14 @@ module ActiveModel
       end
     end
 
+    def self.url(attr)
+      @_urls.push attr
+    end
+
+    def self.urls(*attrs)
+      @_urls.concat attrs
+    end
+
     def self.serializer_for(resource)
       if resource.respond_to?(:to_ary)
         config.array_serializer