avoid exception for general case

app/models/user_first.rb

@@ -5,6 +5,9 @@ class UserFirst < ActiveRecord::Base
   end
 
   def self.create_for(user_id, type, post_id=nil)
+    # the usual case will be that it is already in table, don't try to insert
+    return false if UserFirst.exists?(user_id: user_id, first_type: types[type])
+
     create!(user_id: user_id, first_type: types[type], post_id: post_id)
     true
   rescue PG::UniqueViolation, ActiveRecord::RecordNotUnique