diff --git a/lib/active_model/serializer.rb b/lib/active_model/serializer.rb
index 14220ed0..00a5d591 100644
--- a/lib/active_model/serializer.rb
+++ b/lib/active_model/serializer.rb
@@ -138,10 +138,8 @@ module ActiveModel
           # to determine the serializer
           if options[:key] && !options[:serializer]
             options[:serializer] = const_get("#{options[:key].to_s.camelize.singularize}Serializer")
-          elsif !options[:serializer] && klass == Associations::HasMany
-            options[:serializer] = const_get("#{attr.to_s.camelize.singularize}Serializer")
           else
-            options[:serializer] ||= const_get("#{attr.to_s.camelize}Serializer")
+            options[:serializer] ||= const_get("#{attr.to_s.singularize.camelize}Serializer")
           end
 
           klass.new(attr, options)