Associations doesn't depend on the source serializer anymore :)

lib/active_model/serializer/associations.rb

@@ -2,9 +2,8 @@ module ActiveModel
   class Serializer
     module Associations #:nodoc:
       class Config #:nodoc:
-        def initialize(name, source, options={})
+        def initialize(name, options={})
           @name = name
-          @source = source
           @options = options
         end
 
@@ -13,10 +12,6 @@ module ActiveModel
           serializer.is_a?(String) ? serializer.constantize : serializer
         end
 
-        def source_serializer
-          @source
-        end
-
         def key
           options[:key] || @name
         end