Concurrency problems with Rails' ActiveRecord

consistency_problem.rb

class A < ActiveRecord::Base

  # has attribute "value"

end

class B < ActiveRecord::Base

  has_many :as

  def calc_b
    result = Array.new
    as.each do |a|
      result << a.value
    end
    result
  end

end

class SomeController

  def index
    @as = A.all
    # assume here happends a database write
    @b = B.first
    # now @b.calc_b is inconsistent to the values of the objects in @as
  end

end