gistfile1.ru

    def transfer_credits_to(target, amount, message = nil)
      debit = Credit.new(:amount => amount * -1, :message => message)
      debit.source = target

      debit_params = {"_id" => self.id, "credits_amount_sum" => {'$gte' => amount}}
      debit_attrs = {
          '$push' => 
            {'credits' => debit.raw_attributes}, 
          '$inc' => 
            {'credits_amount_sum' => debit.amount}
        }
      result = self.class.collection.update(debit_params, debit_attrs, :safe => true, :multi => false)
      
      unless result[0][0]['updatedExisting']
        raise "#{self.class.name} doesn't have enough credits to transfer"
      end

      credit = Credit.new(:amount => amount, :message => message)
      credit.source = self
      credit_attrs = {
          '$push' => 
            {'credits' => credit.raw_attributes}, 
          '$inc' => 
            {'credits_amount_sum' => credit.amount}
      }

      result = target.class.collection.update({'_id' => target.id}, credit_attrs, :safe => true, :multi => false)

      unless result[0][0]['updatedExisting']
        debit_params.delete('credits_amount_sum')
        debit_attrs['$pull'] = debit_attrs.delete('$push')
        debit_attrs['$inc'] = credit.amount

        self.class.collection.update(debit_params, debit_attrs, :safe => true, :multi => false)

        raise "#{target.class.name} #{target.id} doesn't exist in the database"
      end

      true
    end