FactoryGirl after_create method does not save

I have a simple situation setup to learn testing with FactoryGirl. The Bank has many transactions. Each time a transaction is created, it must subtract the transaction amount from the total amount of the bank.

Here is the code:

# Bank.rb - Has many transactions
class Bank < ActiveRecord::Base
  has_many :transactions
end


# Transaction.rb - Belongs to a bank and should decrement the bank total when created.
class Transaction < ActiveRecord::Base
  belongs_to :bank
  after_create :decrement_bank_amount

  def decrement_bank_amount
    bank.decrement!(:amount, amount) if bank
  end
end


# factories.rb - Create default factories for testing. This is FactoryGirl 4 syntax
FactoryGirl.define do
  factory :bank do
    sequence(:name) { |n| 'Bank ' + n.to_s }
  end

  factory :transaction do
    sequence(:title) { |n| 'Payment ' + n.to_s }
    bank
  end
end


# Transaction_spec.rb - Creates a bank and a transaction.
require 'spec_helper'

describe Transaction do
  describe ".create" do
    context "when a bank is set" do
      it "decreases the bank amount" do
        bank = FactoryGirl.create(:bank, :amount => 1000) do |b|
          b.transactions.create(:amount => 250)
        end

        bank.amount.to_i.should eq 750
      end
    end
  end
end

The test continues to fail, and the amount of the bank returns 1000 instead of the expected 750. I'm at a standstill!