class ModelObserver < ActiveRecord::Observer
  observe ActiveRecord::Base
  
  # Set this to make the change observer ignore all model attribute changes
  cattr_accessor :_ignore
  self._ignore = false
  
  cattr_accessor :object_arrays
  self.object_arrays = []
  
  class << self
    # All changes to model attributes will be ignored while the given block is executed
    def ignore(&block)
      with_ignore(true, &block)
    end
    
    def with_ignore(value)
      original_value = _ignore
      self._ignore = value
      yield
    ensure
      self._ignore = original_value
    end
    
    def collect
      result = []
      object_arrays << result
      yield
      result
    ensure
      object_arrays.delete(result)
    end
    
    def collect_exclusively
      result = []
      original_object_arrays = object_arrays
      self.object_arrays = [result]
      with_ignore(false) { yield }
      result
    ensure
      self.object_arrays = original_object_arrays
    end
    
    def process(*objects)
      return if _ignore || object_arrays.empty?
      
      objects = objects.select(&:modified?)
      
      object_arrays.each do |array|
        array.concat(objects)
      end
    end
  end
  
  def after_create(object)
    self.class.process(object)
  end
  
  def after_update(object)
    self.class.process(object)
  end
end

class ActiveRecord::Base
  class << self
    def transaction_with_model_observer(&block)
      result = nil
      
      objects = ModelObserver.collect_exclusively do
        result = transaction_without_model_observer(&block)
      end
      
      ModelObserver.process(*objects)
      result
    end
    
    alias_method_chain :transaction, :model_observer
  end
end