For increment/decrement blog post

executed_sql.sql

BEGIN;
UPDATE inventories SET inventory_amount = 99;
COMMIT;

incr_decr.rb

  def atomic_change(diff)
    # gotta lock to prevent race conditions and stale reads
    s = Model.find(self.id, :lock => true)
    # sadly, we need to do our own error checking and validation using this method
    # s.inventory_quantity SHOULD not be stale, but very well could be. Consider this a belt and
    # suspenders approach
    errors.add(:quantity, "cannot be negative!") and return false if s.quantity+ diff < 0
    #the update releases the lock above
    Model.where(:id => self.id).update_all(["quantity = quantity + ?", diff])
  end