org.hibernate.HibernateException message re-associated object has dirty collection

I have an application that does the following steps:

1. Place the object in the session:

   def product = Product.get(1)
   session["product"] = product
   

2. Execute and Ajax call to update the 1-m relationship, then render a partial template showing the new benefit. These should not be saved because the user may change their mind, so it is called "discarding":

   def product = session["product"]
   
   if ( !product.isAttached() ) {
      product.attach()
   }
   
   product.addToBenefits( new Benefit( title: "xx" ) )
   
   product.discard()
   
   session["product"] = product
   

3. Attempt to save the object with a save operation.

   def product = session["product"]
   
   if ( !product.isAttached() ) {
        product.attach()
   }
   product.save()
   

At this point we get the following exception:

org.springframework.orm.hibernate3.HibernateSystemException: reassociated object has dirty collection; nested exception is org.hibernate.HibernateException: reassociated object has dirty collection

Is there anyway to prevent this from happening so that I can re-attach the object and save it, thus persisting the changes to the product returns collection?