gistfile1.txt

NB: A collection has many series, a series is part of one collection.

This doesn't work.  After reloding the collection, it still has the same number of series:

 > c = ArchivalCollection.find("arc:test")
 => #<ArchivalCollection pid:"arc:test", title:"Test Collection", format:"Collection"> 
 > c.series.count
 => 3 
 > c.series.pop
 => #<ArchivalComponent pid:"arc:testref3", title:"Series 1: Sample Series"> 
 > c.save
 => true 
 > c.series.count
 => 2 
 > c = ArchivalCollection.find("arc:test")
 => #<ArchivalCollection pid:"arc:test", title:"Test Collection", format:"Collection"> 
 > c.series.count
 => 3 

This works by setting the parent collection to nil on the series (child).  
Here, I just arbitrarily delete the last series object from the collection:

 > c = ArchivalCollection.find("arc:test")
 => #<ArchivalCollection pid:"arc:test", title:"Test Collection", format:"Collection"> 
 > c.series.count
 => 3 
 > c.series.last.collection = nil
 => nil 
 > c.series.last.save
 => true 
 > c.save
 => true 
 > c = ArchivalCollection.find("arc:test")
 => #<ArchivalCollection pid:"arc:test", title:"Test Collection", format:"Collection"> 
 > c.series.count
 => 2