create_users.rb

class CreateUsers < ActiveRecord::Migration
  def change
    create_table :users do |t|
      t.string :username
      t.string :authentication_token
      t.string :password_digest

      t.timestamps null: false

      t.index :authentication_token, unique: true
    end
  end
end

user.rb

# app/models/user.rb
class User < ActiveRecord::Base
  after_create :generate_authentication_token!

  has_secure_password

  private
  # Generate a session token
  def generate_authentication_token!
    self.authentication_token = Digest::SHA1.hexdigest("#{Time.now}-#{self.id}-#{self.updated_at}")
    self.save
  end
end