when delete co-author relation, the co-author will be reset nil

db/seeds.rb

@@ -33,7 +33,7 @@ FactoryGirl.define do
       { zh_tw: "#{name_tw[n]}",
         en: "#{name_en[n]}" }
     end
-    f.sequence(:type) do |n| "#{type[n%5]}" end
+    f.sequence(:co_author_relations_id) do |n| "#{CoAuthorRelation.all[n%type.size].id}" end
     f.sequence(:email) do |n| "#{email[n]}" end
     f.name_id BSON::ObjectId('4f45f3b9e9d02c5db9000067') #user_id, this is Chris' account
   end