title: Destroy embedded documents based on a field set by attar_accessor
body: <p>Following classes are defined in my rails application:</p>
  
  <pre><code>class Letter
      embeds_many :dispatches
  
      def evaluate
          self.dispatches.destroy_all(marked: true)
      end
  end
  
  class Dispatch
      embedded_in :letter
  
      attr_accessor :marked
  end
  </code></pre>
  
  <p>During my program execution some dispatches will be marked, i.e. I will set "marked" attribute to true.</p>
  
  <p>When I call the evaluate method on a Letter instance, I expect the marked dispatches embedded in this letter to be destroyed. But nothing happen...</p>
  
  <p>Is it possible to pass attributes created by attar_accessor to destroy_all method?</p>