Move responsibility of setting a serializer class and building a serializer for an association to the association class

lib/active_model/serializer.rb

@@ -159,18 +159,7 @@ end
 
     def serialize(association)
       associated_data = send(association.name)
-      if associated_data.respond_to?(:to_ary) &&
-         !(association.serializer_class &&
-           association.serializer_class <= ArraySerializer)
-        associated_data.map { |elem| association.build_serializer(elem).serializable_hash }
-      else
-        serializable_obj = association.build_serializer(associated_data).serializable_object
-        if !(association.serializer_class && association.serializer_class <= ArraySerializer)
-          serializable_obj = [serializable_obj]
-          serializable_obj.compact!
-        end
-        serializable_obj
-      end
+      association.build_serializer(associated_data).serializable_object
     end
 
     def serialize_ids(association)

lib/active_model/serializer/associations.rb

@@ -29,6 +29,13 @@ module ActiveModel
 
       def serializer_class=(serializer)
         @serializer_class = serializer.is_a?(String) ? serializer.constantize : serializer
+
+        if @serializer_class && !(@serializer_class <= ArraySerializer)
+          @options.merge!(each_serializer: @serializer_class)
+          @serializer_class = ArraySerializer
+        else
+          @serializer_class ||= ArraySerializer
+        end
       end
 
       def embed=(embed)
@@ -37,8 +44,7 @@ module ActiveModel
       end
 
       def build_serializer(object)
-        @serializer_class ||= Serializer.serializer_for(object) || DefaultSerializer
-        @serializer_class.new(object, @options)
+        @serializer_class.new(Array(object), @options)
       end
 
       class HasOne < Association