Datamapper DataMapper

I have been messing around with the dm-chunked_query library, and it seems that when you have code like this, the Immutable error is raised, although all the subsequent examples seem fine:

An error:

Model.all(:fields => [...]).chunks(20) do |chunk|
keys = chunk.first.attributes.keys
chunk.each do |row|
row_keys = keys.collect{|k| row.attributes[k]}
end
end
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/Library/Ruby/Gems/1.8/gems/dm-core-1.0.2/lib/dm-core/resource/state/immutable.rb:9:in `get': Immutable resource cannot be lazy loaded (DataMapper::ImmutableError)

This works:

chunk = Model.all(:fields => [...]).chunks(20)
keys = chunk.first.attributes
keys.collect{|k| chunk.last.attributes[k]}

As does this:

chunk = Model.all(:fields => [...]).chunks(20)
keys = chunk.first.attributes
row = chunk.last
keys.collect{|k| row.attributes[k]}