Rails validate uniqueness fails on concurrent inputs

Question

I have a Ticket model with basic uniqueness validation on a string attribute ticket_number

validates :ticket_number, uniqueness: t         


        

Answer 1

@Yule answer was very helpful , it explained Why this happened and that adding database level constraint will prevent it.

The docs says, this will throw an ActiveRecord::RecordNotUnique exseption which i don't want the users to see ofcourse, so here is exactly how to apply the workaround suggested by @Yule:

1. Add an index on the attribute

   add_index(:tickets, :ticket_number, :unique => true)

2. Adjust the controller#create action to catch the db exception:

def create
 @ticket = Ticket.new(ticket_params)
 if @ticket.save
   flash[:success] = 'Ticket was successfully created.'
 else
   render action: 'new'
 end
 #catch the exception
 rescue ActiveRecord::RecordNotUnique
   flash[:danger] = 'The ticket number you entered is already taken !'
   render action: 'new'
end

Same should be done for controller#update action ofcourse

Answer 2

This is very uncommon and you're probably very unlucky that it has, it is possible.

Read: https://github.com/rails/rails/blob/master/activerecord/lib/active_record/validations/uniqueness.rb#L165

Consider the following: User A submits form

• User A submits form
• Rails checks database for existing ID for User A- none found
• User B submits form
• Rails checks database for existing ID for User B- none found
• Rails Saves user A record
• Rails saves user B record

All this has to happen within milliseconds but it is technically possible.

I'd suggest adding a constraint at the database level (primary key).