Add support for if/unless on attributes.

lib/active_model/serializer/attribute.rb

@@ -1,6 +1,6 @@
 module ActiveModel
   class Serializer
-    Attribute = Struct.new(:name, :block) do
+    Attribute = Struct.new(:name, :options, :block) do
       def value(serializer)
         if block
           serializer.instance_eval(&block)
@@ -8,6 +8,33 @@ module ActiveModel
           serializer.read_attribute_for_serialization(name)
         end
       end
+
+      def included?(serializer)
+        case condition
+        when :if
+          serializer.public_send(condition)
+        when :unless
+          !serializer.public_send(condition)
+        else
+          true
+        end
+      end
+
+      private
+
+      def condition_type
+        if options.key?(:if)
+          :if
+        elsif options.key?(:unless)
+          :unless
+        else
+          :none
+        end
+      end
+
+      def condition
+        options[condition_type]
+      end
     end
   end
 end

lib/active_model/serializer/attributes.rb

@@ -17,6 +17,7 @@ module ActiveModel
         def attributes(requested_attrs = nil, reload = false)
           @attributes = nil if reload
           @attributes ||= self.class._attributes_data.each_with_object({}) do |(key, attr), hash|
+            next unless attr.included?(self)
             next unless requested_attrs.nil? || requested_attrs.include?(key)
             hash[key] = attr.value(self)
           end
@@ -54,7 +55,7 @@ module ActiveModel
         #     end
         def attribute(attr, options = {}, &block)
           key = options.fetch(:key, attr)
-          _attributes_data[key] = Attribute.new(attr, block)
+          _attributes_data[key] = Attribute.new(attr, options, block)
         end
 
         # @api private