@target should always be an array

activerecord/lib/active_record/associations/association_collection.rb

@@ -354,12 +354,13 @@ def load_target
         def add_to_target(record)
           callback(:before_add, record)
           yield(record) if block_given?
-          @target ||= [] unless loaded?
+
           if @reflection.options[:uniq] && index = @target.index(record)
             @target[index] = record
           else
             @target << record
           end
+
           callback(:after_add, record)
           set_inverse_instance(record)
           record