no assumptions on the length of the m:m target collection
no assumptions on the length of the m:m target collection

* Maybe this will never be necessary, but I'm still thinking
  about how this whole #assign_or_mark_for_destruction method
  could be refactored. I don't like the fact the name itself
  already implies that this method has 2 different purposes.
snusnu/dm-accepts_nested_attributes