Add support for if/unless on associations.

lib/active_model/serializer/associations.rb

@@ -88,6 +88,7 @@ module ActiveModel
 
         Enumerator.new do |y|
           self.class._reflections.each do |reflection|
+            next unless reflection.included?(self)
             key = reflection.options.fetch(:key, reflection.name)
             next unless include_tree.key?(key)
             y.yield reflection.build_association(self, instance_options)

lib/active_model/serializer/reflection.rb

@@ -35,6 +35,18 @@ module ActiveModel
         end
       end
 
+      # @api private
+      def included?(serializer)
+        case condition_type
+        when :if
+          serializer.public_send(condition)
+        when :unless
+          !serializer.public_send(condition)
+        else
+          true
+        end
+      end
+
       # Build association. This method is used internally to
       # build serializer's association by its reflection.
       #
@@ -79,6 +91,20 @@ module ActiveModel
 
       private
 
+      def condition_type
+        if options.key?(:if)
+          :if
+        elsif options.key?(:unless)
+          :unless
+        else
+          :none
+        end
+      end
+
+      def condition
+        options[condition_type]
+      end
+
       def serializer_options(subject, parent_serializer_options, reflection_options)
         serializer = reflection_options.fetch(:serializer, nil)