You have a nil object when you didn't expect it!
The error occurred while evaluating nil.readonly?
/Users/williamsmith/.gems/gems/dm-core-0.10.2/lib/dm-core/associations/one_to_many.rb:305:in `inverse_set'
/Users/williamsmith/.gems/gems/dm-core-0.10.2/lib/dm-core/associations/one_to_many.rb:299:in `resource_removed'
/Users/williamsmith/.gems/gems/dm-core-0.10.2/lib/dm-core/collection.rb:1237:in `resources_removed'
/Users/williamsmith/.gems/gems/dm-core-0.10.2/lib/dm-core/collection.rb:1237:in `each'
/Users/williamsmith/.gems/gems/dm-core-0.10.2/lib/dm-core/collection.rb:1237:in `resources_removed'
/Users/williamsmith/.gems/gems/dm-core-0.10.2/lib/dm-core/collection.rb:636:in `replace'
/Users/williamsmith/.gems/gems/dm-core-0.10.2/lib/dm-core/associations/one_to_many.rb:177:in `replace'
/Users/williamsmith/.gems/gems/dm-core-0.10.2/lib/dm-core/associations/one_to_many.rb:75:in `set'
/Users/williamsmith/.gems/gems/dm-core-0.10.2/lib/dm-core/associations/one_to_one.rb:31:in `set'
/Users/williamsmith/.gems/gems/dm-core-0.10.2/lib/dm-core/associations/relationship.rb:508:in `feed_item='
./spec/models/memory_spec.rb:40: