6.2 the uniqueness caveat when saving into database: adding index to tables!!!

using validates :email, :uniqueness => true 

 

doesn't guarantee uniqueness!!!!!

 

here is why:

 

1. Alice sign up with address [email protected]

2. Alice accidentally clicks on "Submit" twice, sending two requests in quick succession.

3. the following sequence occurs:  request 1 creat a user in memory that passes validation, request 2 does the same, request 1's user gets saved, request 2's user gets saved.

4. two records with same email address in database.

 

this will probably happen in a scaling server.

 

how to solve it????

 

easy, you just need to enforce uniqueness at the database level!!!

our method is to creste database index on the eamil column, and then require that the index be unique!!

 

so we need to update our data model using a new migration, 

 

rails generate migration add_email_uniqueness_index

 

this will creste an empty file, we need to fill in it:

 

 

class AddEmailUniquenessIndex < ActiveRecord::Migration

  def self.up
    add_index :users, :email, :unique => true 
  end

  def self.down
    remove_index :users, :email
  end

end

 

then rake db:migrate

 

(adding index not only solve the uniqueness problem, it also solve another efficiency problem:

when searching by email column

find_by_email("[email protected]")

 

)