Usernamegen ActiveRecord example model and rake task

codename.rb

# create_table "codenames", force: true do |t|
#   t.integer  "user_id"
#   t.string   "name"
#   t.datetime "created_at"
#   t.datetime "updated_at"
# end
# add_index "codenames", ["name"], name: "index_codenames_on_name", unique: true, using: :btree
# add_index "codenames", ["user_id"], name: "index_codenames_on_user_id", unique: true, using: :btree

class Codename < ActiveRecord::Base
  class CodenameAllocRaceConditionError < Exception ; end
  belongs_to :user, optional: true
  scope :available, -> { where(user_id: nil) }
  scope :allocated, -> { where.not(user_id: nil) }
  validates :name, uniqueness: { case_sensitive: false }

  def self.aquire(user)
    # pessimistic locking to avoid race conditions
    codename = nil
    ActiveRecord::Base.uncached do
      begin
        codename = available.first
        return nil unless codename
        codename.with_lock do
          if codename.user_id
            raise CodenameAllocRaceConditionError
          else
            codename.update!(user: user)
          end
        end
        codename
      rescue CodenameAllocRaceConditionError
        retry
      end
    end
    user.id == codename.try(:user_id) ? codename : nil
  end

  def release!
    update!(user_id: nil)
  end
end

codenames.rake

namespace :codenames do
  desc 'Import codenames from namegen pool'
  task :import => :environment do
    cwas = Codename.available.count
    puts "#{cwas} in pool (before)"
    Usernamegen.all.each do |name|
      Codename.where(name: name).first_or_create!
    end
    cnow = Codename.available.count

    puts "#{cnow} (+#{cnow-cwas}) in pool (after)"
  end
end