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

using validates :email, :uniqueness => true?

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doesn't guarantee uniqueness!!!!!

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here is why:

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1. Alice sign up with address alice@wonderland.com

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.

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this will probably happen in a scaling server.

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how to solve it????

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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!!

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

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rails generate migration add_email_uniqueness_index

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

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class AddEmailUniquenessIndex < ActiveRecord::Migration

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

  def self.down
    remove_index :users, :email
  end

end

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then rake db:migrate

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

when searching by email column

find_by_email("abcd@abcd.com")

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)

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