sti.rb

class Account < ActiveRecord::Base
  def withdraw(amount)
    # ...
  end
  
  def deposit(amount)
    # ...
  end
  
  def close!
    update_attribute!(closed_at, Time.current)
  end
end

# These three types of accounts inherit from a common base class
# so they can all share a common `#close` method.  However, each
# class has its own class-specific methods.

class CorporateAccount < Account
  def close!
    inform_share_holders!
    super
  end
end

class SmallBusinessAccount < Account
  def close!
    inform_mom_and_pop!
    super
  end
end

class PersonalAccount < Account
  def close!
    inform_dude_or_dudette!
    super
  end
end

# Rails is smart enough to detect STI if we add a `type` column
# in the `accounts` table.

class CreateAccounts < ActiveRecord::Migration
  def change
    create_table :accounts do |t|
      t.string    :type
      t.float     :balance
      t.string    :tax_identifier
      t.date_time :closed_at
      # ... more column fields #
      t.timestamps
    end
  end
end

# Creation of a corporate account will create a new record in the
# `accounts` table, but with `type` equal to  "corporate".
CorporateAccount.create(name: "Evil Corp", tax_identifier: "6664201369" ...)